At the start, I had no idea which one(s) I wanted to depict, so I started doodling and quickly ended up with a few messy pages of chaotic little sketches to get (re)acquainted with some of the characters:
I liked little Cruella with her cigarette, and Dr. Facilier with a missing baby-teeth gap, but I really went for the bug-eyed infant Ursula, the sea witch from Little Mermaid. At first I put her in a walker, with all those tentacles sticking out, but decided later to put her in a baby pool, complete with trident rattle and seashell pacifier—the evil-queen-of-the-sea-in-training.
I enlarged my little pencil sketch, cleaned it up, and imported it into Photoshop as a template to paint over:
As the painting progressed, I decided to add Ursula's eel minions, Flotsam and Jetsam (also as babies), sketching them first:
The basic shapes were drawn and filled in with solid colors (following the character's colors in the movie), then shaded with a digital airbrush effect. Here is the final design, as submitted to Threadless: