The Disguiser is a very powerful manipulator that is able to kill a player and steal their identity. This can be especially useful if a certain player is known to be a reputable Town role, such as the Mayor, an Investigator, or a Sheriff.
If there is a Janitor in play, it is frequently a good idea to have the Mafia not kill for one night so a Disguiser can claim his target and either have the Janitor clean the target or use his personal cleaning if it is enabled.
If there is no Janitor and the Disguiser has personal cleaning enabled, have the Mafia kill normally as the Disguiser claims a target. Smart towns will notice that someone was cleaned and that the Disguiser has made his move.
If there is no Janitor and the Disguiser does not have personal cleaning enabled, it may be wise to have the Mafia not kill the same night the Disguiser takes action, so the Town doesn't know whether the Disguiser has made a move yet.
If the Mafia knows a target will be healed, then both the Disguiser and Mafioso/Godfather can attack the target, which will negate the Doctor heal and allow the Disguiser to assume his victim's identity.
It is generally not advisable to use the Disguiser's ability early on in the game as the Disguiser will unable to do anything more for the rest of the game besides be an extra vote. However, if there is a revealed Sheriff or Investigator, disguising as someone else can still sow confusion amongst the town, as they will think the Disguiser's logical target (as they may see two Mafia deaths in a single night) would be that role, when really it is not. The Town might no longer trust that role, or even lynch him, netting you two kills while remaining safe.
If you are discovered by an investigative role, do your best to lie your way out of the situation to survive to the next night so that you may disguise. Good lies can involve claiming to be a Jailor or Bus Driver, whereupon you claim that you will prove yourself tonight as those roles by either jailing someone or bus driving two people, as the Town will believe you as you cannot fake these things. If the lie is successful, you can now evade lynching. Even if you are unfortunately discovered the next day, you have granted your fellow Mafia members one more night to kill, which can sometimes decide the outcome of a game.
It is usually an extremely poor decision to disguise as a revealed Mayor. If cleaning is off then you will be instantly found, as your corpse will appear to be the Mayor and everyone will know the "Mayor" is actually a Disguiser. If cleaning is on, you have a bit better of a chance, but often you will be asked to display your voting power after a cleaned body comes up, which will almost positively lead to the death of you. Don't do it.
The best role you can disguise as is a revealed Marshall if cleaning is on. Not only will you take a confirmed Town role, but there is no way to reliably prove you're Disguiser as asking a Marshall to initiate a multi-lynch for the sole purpose of confirming if he's a Disguiser or not is unheard of. Most Town will naturally assume you're not scum for the remainder of the game, leading to an easy victory.
When disguising, the Disguiser will appear to die the following morning,
If the option "Cleans the target's role" is enabled.
The message "We could not determine the player's role" will appear.
The Disguiser will be told their target's Last Will.
If the option "Cleans the target's role" is NOT enabled.
The target's role will appear as belonging to the Disguiser's previous name.
The Disguiser's own Last Will be left with his older name's dead body. Making for some interesting ways of getting Town to lynch people, for example disguising as a Sheriff and leaving a fake Last Will saying someone is evil.
Disguisers retain their role and have no abilities after disguising as someone.
When disguising as the Mayor, the Disguiser will not retain the extra votes.
Disguisers will not participate in Mason or Cultist night chat but will be in their allies list, therefor disguising as a very talkative Cultist or Mason is generally a bad idea.
If a Disguiser wants a Janitor to clean his target, the Janitor should select the same target at night and not the Disguiser's name.