If available to them, players can edit what is shown in a Death Note using the button at the top of the screen, looking like a bloody piece of paper.
A Last Will and Death Note may hold up to 400 characters each. If a player dies, the Death Note can be reopened and reviewed at any time by selecting their name in the graveyard area.
If you had killed multiple victims in the same night (e.g. due to an Arsonist), your Death Note will appear for each of them. Additionally, you can change your Death Note while the victims are being announced in the morning.
Death Notes are a good way of circulating info while staying anonymous. This can be used by any Killing role.
Sometimes a Death Note can include if a player has a higher defense value than said attacker's attack value, to make sure the person who tried to kill said person isn't openly announced (e.g. the Godfather attacks Player A, but Player A's defense value is too high. The next night, the Godfather kills Player B, and the Death Note includes that Player A has a higher defense value than the Godfather's attack value, and therefore suspicious).
Sometimes the Mafia may frame someone by saying, "Our Consigliere says [insert player's name here] is the Arsonist!" or, "Joe has a higher Defense value than my Attack value." in their Death Note.
Sometimes, a way of implicating others is by putting a player name in the Death Note. Most will probably realize this is not the real killer, but it throws a lot of suspicion around.
Death Notes can confirm what your intended action was during the night if you worry about Witches or Transporters altering your play.
For cocky and risky players sometimes putting their own name in their Death Note is a viable strategy. However, this will most likely lead to questioning and death by a Vigilante or lynching. Death Notes are highly untrustworthy but experienced players (that are evil) can manipulate them to further their own lives, manipulate other Townies, destroy other roles with a higher Defense value than their Attack value and most obviously throw off the game.
Sometimes, the killer will actually be quite helpful and leave clues about who they are or who they will kill next which if after looking at all the people they have killed you can enlist a Bodyguard (or Crusader/Trapper) to protect them in order to kill a mass murdering spree. They may use a certain speech pattern or misspell specific words. However, beware of a killer planting false information.
Death Notes are still quite an important part of the game. This is because a person's role can often be found through Death Notes or imitated. Experienced Mafia or NeutralKilling roles can impersonate someone and ensure that they are suspected. More experienced players may completely disregard it but they help prolong the discussion making it easier for mafia to survive.
A player should never use a Death Note as a "calling card" (if they have acquired a killing role multiple times in consecutive games within a short time frame) as Town roles that are involved in two or more consecutive games with the same player may link a previously used Death Note with the corresponding skin, house, pet and/or name of the player and call them out.
Death Notes can be used by a NeutralKilling role to try to get suspicion off you. If people are accusing you of being a Serial Killer or something of that nature, kill someone that night and put in your Death Note, "[your name] is the Serial Killer," though more experienced players may call you out on the double bluff.
Deliberately misspelling your name helps divert suspicion from you, especially if that name is complicated (i.e. Kharnam Darhenn)
Same goes for the Mafia: If you are the Godfather, writing a Death Note that reveals that you killed the person is not a good idea either, due to the fact if the Mafioso is role blocked, it will display your Death Note, and might give away that the Mafioso was role blocked.