It basically works like a combination of a friending meme and a kink meme:
1. Leave a comment with a brief description of what you'd like to write. Favorite characters, tropes, pairings, which parts of canon you know well enough and which parts you haven't seen at all, that sort of thing.
3. Write short fics based on the prompts you've been given.
4. Read other people's fics. Comment. Enjoy!
If you'd rather draw sketches or make icons/graphics instead of (or in addition to) fics, that works too. Just say so in your introductory comment =)