If you've written at least five 500 word or seven 100 word stories, you qualify to sign up. As I said in my last post, I'm not at all officially involved with Remix but it is probably my favorite fandom challenge, and I would definitely encourage everyone to check it out if they're interested!
There will also later be a Remix Madness round that everyone can participate in, even if they didn't sign up for the main challenge. Also, there's a Mandatory Minimums community with prompts for those who might want to write a few last minute ficlets in order to qualify. I put up a few Breaking Bad prompts, and any AO3 member can also add prompts or fulfill them.