Let me start by prefacing that this is not the only way to play Survivor. The way I go about it is fun, it ensures you're a central figure in your tribe and the game, it cements you as a big name and a power player... but in some ways, that makes merge more difficult. It also takes a ton of time and effort to pull off well. That said, I think it's perhaps the single most reliable way to make merge. So without further ado,
How I play survivor, early-midgame edition:
One of the beautiful things about survivor is how difficult merge gameplay is. In your average Survivor merge, figuring out who you can trust is extremely difficult, and this is because the game ends with a jury vote. That means, for you to win, everyone who will beat you in a jury vote must die. But this is a double-edged sword, because if you're a strong social player, you become a jury threat by default, and (almost) inevitably, at least a couple of your allies will realize they can't let you live to F2/F3. This means long term alliances can break down, blindsides happen, and the game becomes a frantic struggle to survive just one more round.
However, these are all worries for the merge. The good news is, tribes are so much easier to navigate! Anyway, just a quick note on the structure here; Rule 0 is the single most important point I have to offer, Rules 1x focus on early game (pre tribe-swap) play, and Rules (2x) focus on midgame (post tribe-swap, pre-merge) play. Finally, you'll note there's nothing in here about how to play a UTR (under the radar) style game, and that's because I can't pull that off to save my life.
Rule 0: Talk to everyone as much as possible.
If there's a single thing I could hammer into every new survivor player's brain, it's this. Without talking to people, you can't form bonds, you can't make allies, and you practically volunteer yourself as an easy elimination. If someone isn't talking to you, there is zero chance they are on your side. (Quick note, the reverse is NOT true. It's very possible to talk to someone a ton while plotting their downfall.) And by the same token, if you don't talk to someone, they'll realize you aren't interested in working with them, and they'll be perfectly willing to eliminate you when the chance comes.
As for what counts as good PM'ing... each person has their own style. Personally, I favor walls of information; it's the easiest way to have a big, important conversation if you can't be online 24/7 to reply quickly. Other people would rather send a bunch of smaller messages, for fast response and because quantity can be a quality all its own. However, I'd say you should aim to trade at least one decent size (at least a couple paragraphs) PM per player, per day. If for some reason you won't be around for a day or more, make sure you let everyone know (allies and otherwise) ahead of time!
Rule 1a: Talk to everyone as soon as possible.
This starts right at the very beginning of the game. As soon as possible, send PM's to everyone you're allowed to contact. It's nice to make them clever, or add flavor, or what have you, but the important thing is to send them. Let your tribe mates know that you're there, active, and ready to talk. In the very early game, there won't be much to talk about, but make do with what you can. Anything from the game's flavor to discussing the challenge to flat out copying an icebreaker could work. Just get the conversation started, then be yourself.
Rule 1b: Make the shift to game discussion as appropriate
This isn't really hard and fast, but at some point early on, people will start talking more about the game. It often starts with general, innocent, "What do you think of the rest of our tribe?" style questions, but this is the groundwork from which alliances are made. This is the point where people start figuring out who they think is a good ally, someone they can stick with through the tribe and into merge. And you should be doing the same! I'd say by the end of the first challenge, if you're communicating with everyone, you should have a good sense of which players are strong PM'ers, which ones are okay at it, and which are borderline inactive. Once you've got that list, feel free to share the general gist of it with the other players. Survivor players love getting that kind of info, just to check that their read on people is accurate and that they haven't missed anything important. And once this sort of thing starts, that's when the game discussion really opens up. The moment someone mentions an alliance is when the game really begins.
Rule 1c: Get on that majority alliance.
Assume that at some point your tribe will go to tribal council. (There's an outside chance it doesn't happen, but you should always be preparing for the next TC.) In order to be safe, you want to be included in whatever the majority alliance is. This is the first concern of just about everyone in the early game. This is the end to which all of the "So, what do you think of the tribe?" style questioning is actually directed. If you're in a seven man tribe, then what everyone is secretly thinking is "What other three people do I want with me in the majority?". If you're on a twelve man tribe, they're looking for a seven or eight man alliance. (In larger tribes, it's not uncommon to grab an extra person for the majority just in case.)
And if you're on a twenty-four man tribe, please message Xofelf and tell her to cut out the shipwreck phases.
The good news is, if you've been properly responding to your PM's, you've got a very good chance of being on that desirable ally list just by default.In addition, due to that desire to be on the majority alliance, it is extremely rare for people to turn down early game alliance requests. (I've had it happen exactly once in the dozen or so games I've played on this site.) Don't spam them mindlessly, but if you're having a good conversation with someone, and you ask them to ally, they'll probably reciprocate. In addition, if you keep in mind who has a good opinion of whom, it's not too hard to guide the majority alliance into shape. If you know Player X likes Player Y and you like them both, something along the lines of "Hey X, I've also enjoyed talking to Y, do you think we should try to work with them?" often leads to a three-way alliance between you, X, and Y. Rinse and repeat until the desired majority has been reached.
Rule 1d: Don't force things.
In the tribes phase, I strongly recommend not to rock the boat too much. Survivor is a long game, and any enemies you make or deals you break have a lot of time to come back to haunt you. (Sometimes literally, depending on the game's twists.) In particular, you don't know when a tribe swap might happen. That guy who's sort of on the edge of the alliance but not really part of the core group? He might be the only tribe mate you have with you after a surprise swap. The player who was the best ally of the guy you backstabbed to seize power? They might decide the tribe swap is the perfect time to get a little revenge. In your starting tribe, the correct play is almost always the least controversial one. Because of that, the first boot (and potentially the first few boots) of a tribe are typically the inactives. People that don't PM much are players that nobody cares about, and that nobody will get upset over leaving. As long as that's not you, this is a good thing.
Rule 1e: Personal connections are more important than alliance agreements.
People play to win the game. This means that they'll do what's best for themselves over what's best for the alliance. (There are exceptions, but finding someone who will hurt their game for your sake is rare and should not be counted on.) Alliance agreements are still an important tool, don't get me wrong. But don't expect people to just blindly follow them, especially if the situation changes significantly. Something like a tribe swap or a surprise blindside can shatter a large alliance just because the numbers don't work any more. Personal connections, on the other hand, tend to endure better. If someone likes you and wants to work with you, they'll usually try to keep you around, even if they can't openly be on your side. Being able to maintain these is extremely important, because it keeps you from being the target. Winning every TC is extremely difficult, but if you can avoid being the one that gets voted out, you have a chance to regain power, or stall for a twist to keep you alive.
Rule 2a: In a tribe swap, value your original allies, and look for new ones.
So, you've been tribe swapped. Hopefully you have at least some of your original allies still around, but there's also a bunch of new people you haven't talked before. Now what? Start by talking to everyone. Just like the beginning of the game, say hello to everyone, and make sure they know you'll be active. Unlike the beginning of the game, you don't have a ton of time to icebreak with flavor talk. Serious discussion needs to happen right away, because there's immediately important things to talk about. Your goal for this phase of the game should be not just to survive, but also to come out of it in a better position than you went in. This means finding new allies. Being able to make cross-tribal alliances in a game of Survivor is an extremely important skill. However, don't neglect your original allies! They're still on your side, at least in theory. There are times where you just can't make it work and you have to give them up, but if you can, you should try to keep them alive.
Rule 2b: Tribes are a tool. They are not the be-all end-all
Breakdowns along tribal lines are an easy place to start, and an easy thing to throw around. In games with multiple tribe swaps, or more than two tribes, this gives you even more to work with. After all, people who were on the same tribe before at least know each other, and there's a reasonable chance they're allied. However, this is only a starting point. In practice, you'll find that tribal councils form fractures. Maybe that three man group from the other tribe is a really close pair and a third wheel only tangentially aligned. Or maybe the "majority" group from the other tribe is actually two good players and two inactives, and they have no interest in sticking together. There's almost always more underneath the surface if you know where to look. Use the tribe-level analysis as a starting point, a conversation piece, and even an argument for why they should vote the guy you want out, but never be content to just stop there.
Rule 2c: Pagongs are dangerous. They can and do happen, but a sustained pagong requires bad play.
Suppose after our tribe swap, we have a seven man tribe: three people from tribe A, and four people from tribe B. As the four people from tribe B have majority on their own, they could just team up to wipe out tribe A, guaranteeing them safety through three votes and probably the whole phase. This is the tactic known as a pagong, and while more infamous in the merge, it can happen in the tribe swap as well. People hate pagongs. They're boring and offer very little counterplay for the minority. But are they good strategy?
I would argue a pagong is almost always a bad plan. First off, by locking out tribe A, those four players from tribe B have burned their bridges. Any members of tribe A that survive (both those on the tribe and off it) will almost certainly work against them in the future. Secondly, there's a good chance at least one member of tribe B would benefit from jumping ship. A single defector would be enough for tribe A to take control, and if one of those tribe B members was on the outside of the original tribe, getting three good allies from tribe A could be well worth bailing on his poor position from his original tribe.
In general, for a pagong to happen for any length of time, it usually means someone in the majority thinks they're way higher on the totem pole than they actually are. If someone thinks they're on the outside of the majority, they should (and usually do) flip to get a better position.
Rule 2d: First-ally advantage is important, but not the only factor.
When it comes to cross-tribal allies, one thing you need to ask yourself is what sort of alliances they might already have. (This is important pretty much all the time, but particularly now.) And this is because if everything else is equal, most players will value a pre-existing alliance over a new one. But that doesn't always have to be the case. Particularly if you get a longstanding cross-tribal alliance, those bonds have a chance to shift. Generally speaking, if you can make someone feel close to you (by talking with them a bunch over PM's, detailed strategy and the like), they might move to protect you over an earlier ally they don't care as much about. This doesn't work very well with new alliances, but it's something to think about for the long term, and a reason not to slack on communicating with your original tribe-mates!
Rule 2e: If you aren't part of the majority, look for openings in it.
Sometimes things just don't go your way. Maybe your alliance made a bad play. Maybe you were betrayed. Maybe they had a hidden immunity idol. And maybe it was just a really terrible tribe swap. Regardless, all else has failed, and you're on the minority. Now what? Look for cracks. This is more of an art than a science, but it all comes back to the whole establishing connections principle. If you make yourself seem like a possible asset to whoever's on top, maybe they'll keep you around a couple more votes, just in case. If you can buddy up to the people on the edge of the majority, maybe they aren't actually that close and they'll be willing to flip. Maybe your best play is to straight up go on your knees to the group and offer to be their extra vote in exchange for safety.
Rule 2f: Never burn bridges.
Anything can happen in survivor. Anything. Even if you've worked against someone for just about the whole game, it's possible that the exact right twist of fate puts you on the same side of a TC down the road. Up until you see someone's name on the eliminated list, you should keep an open mind about the possibility of working with them, because there's a real chance it could happen.
Rule 2g: Be prepared to spend all night planning the TC
Early TC's on your first tribe can be easy. But once you get to a tribal council where everyone is active, things can explode. Figuring out who the best play is can take a long time and a lot of careful negotiation, and can change several times over the course of a night. You want to be a part of this decision making process until the plan is confirmed and everyone is onboard. Sometimes RL intervenes, and missing part of the discussion is unavoidable, but that can be a game-ending problem. (In my last game, I went from being a potential winner candidate to dead man walking during the span of my normal workday.)
Rule 2h: If someone suddenly stops talking to you (while still talking to others), they're planning something.
Most survivor players will have a lot of difficulty keeping up an in-depth strategic conversation with you if they're planning to vote you off. Part of it's that people only have so much time, and part of it is that lying in detail like that is difficult. Regardless, it's an effective early warning signal if someone has decided to move against you, or even just that you aren't part of their long term plans. And if everyone suddenly stops responding halfway through the tribal council, you're probably doomed. If you have a desperation play available, go for it, because now is the time.