You take a character from a children's book and make a character based off of it. Of course you horribly warp it first. Here's the real challenge the campaign is a more serious, darker world then normal and your characters need to fit in.
I made a character based off Clifford the Big Red Dog. My version had him being the son of Cerberus and had him expelled from hell and onto Earth by his father to save him from his father's enemies. He had fire based powers as a person and when he changed to his dog form (looked nothing like the real Clifford other then being big and mainly red) he was very strong, very fast, and capable of summoning a pack of hell hounds to serve him.
Another player decided to make Emily Elizibeth because I mad Clifford. Her version of Emily Elizibeth was a pretty gloomy 16 year old witch. The game was tons of fun and so popular we may continue to play it.
Jeff gave me an exhaustive description. I hope I did his team justice. To those others that I have commissions to do for, I will be getting to them slowly and surely. Origins is eating up a bit of time due to prep. But I'll be taking sketches to do with me.