Pick a character on the list and ask your opponent yes/no questions to find out which character your opponent chose. When your opponent has answered your question, you will have 5 seconds to click the options that aren't possibilities. When you only have 1 option left, the game will automatically ask the other player if is their character. Press S to show how many options are left, which player is asking, etc. Press H to close it.
Things to keep in mind: When quitting the game, always remember to press the quit button before you exit. Only 2 players can play the game at once. If there are already two people in the game, it'll kick you out. This is still in progress and I'm currently testing this with @bjartur1007 so while it's a test, I'd appreciate it if you didn't play it.