The idea of a remix is that each participant is assigned a writer and you rewrite one of her or his stories. From the rules:
In rewriting, the participant can change any stylistic element they like - point of view, structure, mood, length, language style... just about anything. What they can't change is the fundamental story. What would be the point, otherwise? The basic events, people, and times of the story must stay the same.
You're also allowed to designate one of your own stories as "off limits" to remixing if you want to, and when you sign up you can tell the mod what you absolutely won't write, whether it's a pairing or a character or whatever. msilverstar, who runs the fest, will match you up with someone who'll give you a reasonable variety of stories to choose from, within your stated boundaries.
Check out the community to see previous remixes, which are all linked to the story which inspired them. Note that trianne's Waiting for Viggo, a remix of chaosmanor's story Walking on Sunshine, is up for a MOME award; there's some really good stuff here. :) Come sign up!