What you'll need: Scotch Tape, two polishes that compliment each other, and a top coat polish.
Step 1: Paint all nails with one of the colors. I found it easier to do this with the color you want to be at the top of your nails. Let dry for at least a couple hours, I waited over night, so the tape wouldn't peel off the mint layer later on.
Step 2: Place tape strips diagonally across each finger, creating two equal diagonal halves. Start at the top corner, then ease the tape down to the opposite bottom corner. Fold tape around your finger to keep it neat and secure.
Step 3: Paint the bottom portion under the tape the other color. You may need two coats. It's best to do two thin coats, as opposed to one thick coat. You don't want a big height difference between the two colors.
Step 4: when the bottom color is dry, very carefully peel the tape away.
Step 5: Apply a top coat to keep polish put and even out any height difference.
Easy as pie! You can feel free to clean the edges a bit with a paint brush in nail polish remover, but I'm lazy and skipped that step.
Give it a try and let me know how it goes!