Gather all of your supplies beforehand. For a reciprocal gradient, I like using two very different colors, not 2 shades of the same color.
Start off with a base coat.
Paint the lighter color on your nails. I did two coats to get the opacity I wanted. Let it dry completely. This color is Covergirls "Constant Carribean", for anyone wondering.
Paint some nail polish directly onto the makeup sponge. Overlap your colors just a little bit so you can get a good fading look.
Dab the sponge onto your nail a few times. Do all your nails in layers so they dry at least a little between each coat. Be sure to keep adding more polish or you'll get sponge chunks on your nails!
Wait for the first gradient to dry COMPLETELY! If you don't, the tape will peel up the polish. I put on a coat of seche vite topcoat to speed things along. Put 3 vertical pieces of striping tape on.
Do the same thing as step 5 but with your colors in the opposite direction. If your tips were black, this time the moons will be black. Peel off the tape to reveal your gradient!
Put on a topcoat to smooth things out a bit and clean up around the nails with a qtip soaked in nail polish remover.
And you're done! Ta-Da! That's how simple it is to get this eye-catching look.