I put them on top of a white polish. They are super fun to do. All you really need to do is cut them to the shape of your nails and then peel the plastic off and soak them in water until the pattern slides off the back paper. Very simple.
But when I did it I was alright with 9 and my last one came off on the plastic instead of the paper so... I have one colored cat nail. Hahah. Looked weird just leaving it white.
Anyway you can check them out here: