Choose 2-3 (or more if you're feeling brave!) different color polishes that you think will create a nice gradation. Choosing color near each other on a color wheel will ensure this, but be crazy and out there and choose whatever you'd like.
You will need a plastic surface to paint nail polish onto, so be sure to have some wax paper or plastic sheet. I use an old tupperware lid - anything works!
Be sure to dampen your makeup sponge before getting started.