This past weekend, I got to use Sharpie markers to do artwork on my daughter's nails. If you want to see this done, go to YouTube; lots of great videos there. I watched a few, talked to some fellow nail techs, and just went for it. My supplies consisted of: Fine tipped Sharpie markers, gel polish top coat, IPA99 (99% rubbing alcohol), a small mister bottle, a dotting tool, and a small art brush.
First, I gave her a rebalance, as she wears gel enhancements. We took them from an "Almondetto" shape (longer than traditional Almond, but shorter & wider than a standard Stiletto), to a short "Squoval" shape (squared with slightly rounded corners). I then applied two coats of white gel polish, curing each layer. I *did not* top coat at this point, and realized later than I should have. The Sharpie slide over the nail much nicer with a clean top-coated surface rather than the slightly tacky one I ended up with... lesson learned.
Viewing the pic below, the methods I used were --
A) Squiggle -- using hot pink, purple, and light blue Sharpies -- your colors all over the nail, then use a clean small art brush, dipped in IPA & patted almost dry on a lint-free pad, and squiggle over your colors. You'll want to do this in small sections so as to not meld your colors together too much. Let dry, *carefully* top coat (being careful to float it on top of the nail, not pressing the brush into the art; you'll smear it if you do that) & cure.
B) Squiggle on your colors. Put IPA in a small mister bottle, hold nail about 8" or so from bottle (I did this with the nail horizontal to the floor, with my hand under it, but you can get a slightly different effect if you hold it upright), spritz once or twice, let IPA spread & dry. You can always add more color and spritz again if it doesn't look quite right. Carefully top coat & cure.
C) Squiggle on your colors. Put IPA in a small dish (dappen dish works great, but you can use your IPA's lid if needed), use both the small end & the large end (for different sized dots), dip the dotting tool in the dappen dish, dot once or twice in different spots on nail, let spread, repeat as you see fit. If needed, reapply color, and repeat. Once you get the effect you're looking for, let it dry, top coat, & cure.