I've seen a few different ways to do this:
#1: take a cheap bottle of clear nail polish, dump some loose eyeshadow pigment in, and stir (most tutorials say to stir with a toothpick or something skinny that can get into the corners). This makes a polish you can seal and keep, so it's better for a color you know you'll wear again and again. I would try experimenting with some of the NYX pigments, as they're pretty inexpensive.
#2: Paint your nails with clear polish. Load up an eyeshadow blending brush with pigment, hold it over your nails, and tap til the entire surface is covered, picking up more pigment on the brush if you need to. Allow it to dry for a few minutes, then gently brush/tap/blow the extra pigment off. Seal it in with another coat of clear polish. Once that's dry, you just have to wash your hands to get rid of the extra pigment on your nail beds.
#3: Kind of the same as #2 only instead of a clear polish, paint on a color (say, black). While it's still tacky/drying, place a tiny pile of pigment at the base of your nail (in another color. We'll pretend it's silver). Then, gently blow the pigment towards the tip of your nail, stopping wherever you want. Wait a few minutes to let the polish dry, dust off the remaining pigment, and seal in with a clear coat.
You can find a lot of tutorials by googling "nail polish eyeshadow pigment diy", and every single one of them had some variation in directions, so go out there and find the one that works for you!
FYI the #1 method makes a very matte nail polish color, a few coats of clear polish (not the one you mixed up) will make the color shiny again! Good luck! Post pics if you decide to do any of these!