1.) Paint your nail your lightest color and let dry completely. I used white because my colors were slightly translucent.2.) On a makeup sponge (you can buy these for a few bucks for 30 at a drug store) brush your two colors slightly overlapping. (I used China Glaze Aquadelic and Sunshine Pop. The nice green appeared as I blended)
3.) Roll the sponge from right to left over your nail. It will appear patchy and that's ok. Add new paint strokes to your sponge as needed as you do all your nails. Let this layer dry. (It will dry quickly)
4.) Repeat, painting your colors on your sponge and rolling from right to left in a second coat. It will appear less patchy. Let dry.
5.)Repeat brushing the sponge and rolling from left to right. This time after you've rolled it side to side dab the sponge up and down the nail. This will blend the colors and give it a more natural gradient. If you need more coverage or a smoother gradient repeat this step.
6.) IMPERATIVE! Clean up around the nail and finish with a top coat. This will smooth the gradient and make it look much more natural.
If you're feeling ambitious You can use more than two colors in your gradient.
If you're feeling extra fancy you can even combine techniques like hand painting some nail art over a gradient.
Or try a clear and black water marble over the top.