I've done the last two years. There are no hard and fast rules, every remix is different - the first year I took a drabble and expanded on it, the second year I took a longer fic and wrote it from the other character's POV. This year I think I am going to take a femslash and rewrite it as Dan/Casey. Probably the best thing to do would be to read a few, original and remix, and see what other people did.