To learn how to play the early game you first have to learn how to play the end game; the goal of the early game is to set up a favorable end game and you cannot do that if you do not know what it looks like or how to capitalize on it.
The simplest end game that everyone needs to know about is one impostor vs two crewmates. This should be a won game for the impostor.
Sabotage the reactor, follow the crewmates there but not help in salvaging the crisis, then walk with them back towards the button and kill one person as soon as it is off cooldown.
If the kill cooldown is very long you can waste some time by waiting for the button cooldown to get close to zero before calling the sabotage. Start the round by checking the button cooldown and then count in your head. Do not keep the button open as your kill cooldown does not count down while the button is open. On some maps you can also slow them down by locking doors on the way back to the button.
It doesn't matter if they know that it is you as soon as the round starts, if you play it correctly they cannot win anyways.
One impostor vs three crewmates is a very common scenario that most people struggle with, but as long as the task bar isn't dangerously close to complete it is actually pretty good for the impostor. How to play will vary depending on whether the crewmates walk in a group or not.
If the crewmates split you should follow someone who walks alone. Preferably someone who has a little trust for you. If they press the button because you are following them you can say that you followed them because you wanted to group up together. The best strategy for the crewmates is to walk everyone together so you're almost telling the truth.
When you've isolated one crewmate you want to kill them in an off-the-path area, like med bay or electricity on Skeld. Then you call the game ending sabotage that is closest to you and run there immediately. You want your kill cooldown to tick down so don't stand there with the fix screen open. Whether to help fix it or not once they show up is a judgement call depending on how many of them show up and your kill cooldown. Then follow a crewmate and kill them as soon as the cooldown allows.
You lose if they find corpse or reach the button so its important that you don't kill someone on the straight path to the sabotage, but if it's a side area that branches of from the main path that's fine as they likely won't be checking the side areas while the crisis is ongoing.
The second way you lose is if they fix the sabotage before you get there and then you either lose them or the run straight to the button, which is why you prefer sabotages that are close to you.
The third way you lose is if you help fix the sabotage and one crewmate is camping the button, which is why you don't help fix it if only one crewmate shows up.
Grouped up crewmates
If the crewmates group up you want to wait until you're a good distance away from lights. Preferably you can lead them in such a direction by saying that you have tasks left in such an area, and tell them to follow you. Once a good distance has been achived you sabotage the lights.
It is very unlikely that they will manage to stay perfectly grouped with the lights off and when the opportunity presents itself you kill the straggler. Beware of the group stutterstepping as then they'll likely see the report button even if they don't see the corpse, but if they are running at full speed towards the lights and one guy falls half a screen behind they're a free kill.
After the kill you run with them towards lights and then follow them after you've helped fix it. Their goal is to either find the body or the reach the button before your cooldown is ready again. This is why you want to be far away from lights when you call the sabotage. Lock doors to slow them down if possible.
Whether the crewmates are teamed up or not the key to winning is getting two kills in the same round, which requires using a sabotage to keep them from finding the first body.
If the crewmates pull off the stutterstepping towards the light perfectly there's a backup strategy that is more risky and seldom needed. But what you can do is to fix the lights and then break away from the group when the sabotage is off cooldown again. Then you wait until they are fairly close to a vent (you can use the vents to keep track of them), sabotage the lights, vent in and kill someone and vent out, then help fix lights. In this situation it might actually be best if they see the report button and press it right away (but don't press it yourself, you're pretending to be somewhere else). This is also how you kill if the crewmates group up all together in the early game.
The main ways you lose here is if the task bar is to close to complete, or if your stupid ghost teammate wastes the sabotages on things that aren't lights.
1v4 with reputation advantage
If one crewmate has been called out as suspicious earlier in the game, and you have not, then you should follow them in a one against four situation. As soon as you meet a third person you kill that third person and instantly self report and blame it on the suspicious crewmate.
With one crewmate killed and another voted out you end up in the one against two scenario which is a freewin for you even if they know that you are the impostor when the game doesn't end after the meeting.
Even if they skip on the vote you'll get to play one against three with one crewmate looking very suspicious which is a large advantage for you. As long as you pull this trick against a crewmate that has be sussed earlier they're extremly unlikely to vote you out, even if they don't vote the crewmate out.
Beware of killing the wrong person when there's three of you together. If you kill the suspicious crewmate in front of an innocent crewmate instead of the other way around you're much more likely to get voted out. In public lobbies it is also very important to be quick with the accusation, public lobbies often sides with the first person to make an accusation.
The same trick can be pulled in a one against three situation, but it is less likely that a suspicious person will last so long and that the crewmates will split up on top of that.
If you play repeated games with a regular group you can get creative with the way to make the accusation. If they know that you've pulled the same trick before you can pretend that you didn't see who performed the killing and instead let the suspicious crewmate accuse you and then turn it around on them ("it has to be X, since they are lying"). This is suicide in a public lobby but regular groups allow you to be creative.
Play it like 1v2 but you need a double kill to end it.
2v5 with reputation advantage.
Play it like 1v4 with reputation advantage.
Setting up a game ending double kill
Stick together, perform a game ending sabotage like reactor or O2, kill one person on top of the fix button, or close to it. The second person walks away a bit and guards the body from a distance.
2. Early game
The main goal of the early game is to avoid suspicion while keeping at least one suspicious crewmate in the game. Aggressively call out suspicious behavior in meetings, but also state that you don't think its enough to vote them out yet.
If there's only one suspicious crewmate you should actively try to keep them in game without putting yourself in the line of fire. You can say things like "I don't think there's enough evidence. Focus on tasks but do remember that X is sus." or "Don't vote on 7, but remember that X is sus." You also don't want to kill a suspicious crewmate, instead, actively seek out teammates who have been cleared ("X scanned" or "I've been with Y the whole round"). However, don't defend them if they get accused of venting or killing in front of someone as that just makes you look suspicious for defending them. Similarly, don't defend your teammate if that happens.
General killing strategy
Don't kill where people have seen you enter.
Don't kill people who you have been seen with.
Don't kill suspicous people.
Don't walk away from kills with lights on; vent out if they are on.
Don't kill in hallways with lights on.
When looking for kills, walk around the map alone. If you stumble upon one person or a group and cannot kill them because of one of the reasons above, fake tasks. The standard way to kill someone is to stand on top of someone, sabotage the lights, and then kill them once lights reach 0%. If you your teammate sabotages lights you should actively look for opputunities, don't waste time faking tasks. If you meet someone when running away from a body with lights off, turn around and try to either hide in a side path, or run for a vent. Beware that lights can turn back on at any moment and you might notice as your vision doesn't change, so keep an eye on the task list.
If an opportunity for a kill presents itself when the lights are on and sabotage are on cooldown you should run straight for the vent afterwards, and only kill if there is a nearby vent. Getting caught running away from a body is a sure way to get voted out.
If a meeting happens before you've gotten far enough away from the body, don't call out your positition and don't lie randomly. Try to avoid saying where you've been until enough people have stated their position that you have some idea which areas and paths that you could plausily have taken without having been seen.
3. General Tips
Each sabotage has a unique purpose.
Lights are for getting away after killing and should be your bread and butter.
Reactor is used in the specific end game situations mention above.
O2 is a worse version of reactor (since its closer to the button on every presently released map) that should only be used in very rare variations of the above end game situations. For example, in a one against two situation where the crewmates run directly towards the reactor after the meeting. On Mira O2 is more suited for double kills as its far enough away and its split nature makes double kills easier to set up.
Communications should almost only be used if you're faking the final part of a task and someone seem to be actively watching you. Normally you wait until the task bar moves and then leave but if you've already stood there for a suspiciously long time you can sabotage comms and then move. Sabotaging comms to get rid of the cameras is not worth it as they will already have seen you around the location of the murder. If you're very good and know the sight radius of cameras there may be very rare situations where you can see the camera blinking and kill after sabotaging comms from outside the camera range, but these situations are going to be very rare.
Sabotages require good timing to be useful. Don't randomly sabotage things just to slow down the crewmates and NEVER EVER sabotage as a ghost (except for doors). The ghost wasting sabotages is my number one reason for losing games as impostor.
Once your sabotage has served its purpose, or if your teammate calls a stupid sabotage (like reactor in the early game), you should help fixing it.
Perform the mouse or hand movements that you normally would when performing the task. This makes the timing realistic. If the task is just pressing a button while a bar moves, move your mouse as the bar would normally.
When faking a short task or the final part of a multi part task, wait for the task bar to move before you move. Avoid faking these tasks in the late game as the bar is less likely to move.
If you say that you did the first part of a multi part task during a meeting its a good idea to start the following round by finishing the remaining part(s).
Wires have a specific order, don't fake them in the wrong order. Wiki. Most importantly, don't fake one of the later wires right after the start of the game, nor one of the first wires during the end of the game.
Upload/download takes 8 seconds, med bay scan takes 10 seconds, astroids around 12 if you don't miss. Count in your head.
Don't forget to fake plausible common tasks. E.g. if you have keys on Polus you have to start the game by faking that. If you have have Swipe Card or Scan Boarding Pass on Polus you should fake those after the first meeting, unless someone has seen you fake those during the first round, or unless you said that you were on vitals during the first round. Similarly, don't forget to do Swipe Card or Enter Id Code when entering the admin room in the early game on Skeld/Mira.
If you get sussed during the early game it's often better to lay low and try to befriend a crewmate while letting your teammate perform the killings.
If you walk by admin and its empty you should check it. There may be opporunities to sabotage lights and then vent towards a lone crewmate to kill them.
If you see the cameras blinking when you're in the cafeteria on Skeld you can lock the security doors and vent in from med bay to kill the camera man.
If you're standing in electricity or med bay on Skeld and someone enters you should lock the doors before killing them. You should still sabotage lights before venting out though, as that will help you leave the vents.
On Polus both storage (fuel) and communication (upload) are good places to kill. If you see someone doing the first part of one of those tasks you know they'll likely do the second part soon, and you can be waiting for them there.
Mastering the basics to the degree where you don't have to think about them will let you focus on more advanced stuff, like memorizing what actually happened during the round, discovering weaknesses in crewmate actions and stringing together creative lies.