I first painted my nails with Wet n' Wild White as a base, then I took a sponge (I just use whatever I can get my hands on - I use the sponges that I have at home to wash dishes), and did three stripes of paint in each colour next to each other on the sponge. Then I transferred the polish from the sponge to my nail. I couldn't figure out how to get rid of the "sponge marks" on my mani. Maybe I should have used more polish on the sponge, or a different type of sponge? Anyway, when I figure it out, I'll let you guys know.
The trick to this, I think, is to blend the polishes on the sponge before transferring them to the nail. Also, I think it helps to use polished that are opaque and give good coverage.
The downside to this is that you get polish all over your cuticles and skin around the nail, and cleanup can be a pain. Especially if, like me, you used black or a dark shade, then it's quite hard to remove all that crap that gets stuck at the base of your nails. But the end effect is so cool, I think it's worth it!
So ta-da, my gradient manicure! I'm really loving how this looks, despite the pain it was to clean up, it has a very moody-chic vibe to it.