1. Comment in the post listing your characters.
2. Other will reply with the name of one of your characters
3. You list three things that are your headcanon for that character.
Alternatively, someone can ask you a specific question about the headcanon you have for a certain character. This gives you the option to answer questions pertaining to certain subjects, or think about a question you hadn't considered/answered with your headcanon yet.
There's no limit to the topics or the amount of times someone can ask about a character. It can be huge things like how their life was before their canon, or something smaller like their first time making sweet sweet... um, baked goods for the family. As long as you think it's something that is accurate to your character but it isn't in their canon.