You can choose colors that compliment or go bold and pick two that clash. Experiment with different combinations and see what works.
Your best bet is to get a big bathroom sponge and just cut sections off when You need them. The sections don't have to be perfectly cut, as long as one side is flat it will work. You can use absolutely any sponge - a makeup sponge, washing up sponge or any other kind of sponge you can find. I just blended the edges with the polish brush - just wipe any excess polish if it first.
Using the first of your colors, paint your nails and wait for it to dry completely.
Starting at the tip of the nail, apply the polish using less as you get further down the nail
Apply a top coat & your done !