First I applied a base coat and then one coat of Sally Hansen White On. This coat doesn't have to be perfect, it's just a base so the colours on top will show up better, especially if they're slightly sheer.
Next, I took two polishes per nail and applied them both to seperate parts of my nail, overlapping the colours where they met to give a gradient effect. I continued the next nail with the last colour used on the previous nail to give the effect of a rainbow.
I did the same thing to each nail with the following colours:
Thumb: Sally Hansen Wedded Bliss and Barry M Pale Purple
Index: Barry M Pale Purple and OPI What's With The Cattitude?
Middle: OPI What's With The Cattitude and Barry M Mint Green
Ring: Barry M Mint Green and China Glaze Lemon Fizz
Pinkie: China Glaze Lemon Fizz and w7 Flourescent Orange.
I applied OPI Text Me Text You randomly to parts of my nails to add a bit of sparkle and then used Seche Vite as my top coat.