Ah, now, you knew this was coming, what with the tip and all. Yes, write a Mary Sue. I mean, a hideously BLATANT Mary Sue, one that would make the neighbors gag. And have fun with him or her! I mean, all out, sit there at the PC and think "I can't believe I wrote this" fun! Mary Sues can be fun! Make her as hideously, nasty perfect as you can get. I encourage the use of pink, unicorns and being as famous as Harry Potter...maybe a twin of Draco Malfoy previously thought lost on an island when the twins were 4 and on vacation, raised by natives as a goddess....
...or some such drivel. Me? Mine's gonna be a ROCK STAR!
(giggles madly)
As for what to write, in any format you want. Do a character description, write a scene where she meets some HP character, whatever! Just whatever pops in your head. She doesn't have to be taken seriously, in fact I'd rather she didn't. Let's see how many of us remember our Mary Sueing days. (twitch)