As the imposter your main job is going to be to kill off the crew all while looking as innocent as possible. This could even lead to killing someone in front of another player and blaming that other player. This is called a self report and it's a high risk high reward play style. If you play with a group of friends then this won't work every time as they start to learn your play style and can instantly call you out on it. This is why you want to change up your play style every few games or when the crew start to catch onto your style.
My style of play is usually to stick with a crewmate the entire game so they can vouch for me and if I get the chance then I will stray off from the group, snag a kill, and come back unoticed.
Each map has vents around and the impostor can use these to get around quicker or to just hide if they are in a sticky situation. One thing you must be careful about though is having someone walk in on you hopping in or out of the vent because then they will just go and press the button to report you for venting.
Venting.webm
Vent Kill.webm
This is a vent kill and in my opinion the best way to kill. If you're fast enough you could frame somebody without anyone seeing you. You could even just lead someone to a place where almost nobody goes and then vent out to a more popular area in order to get an alibi.