Start with 3 black nails and 1 white nail. After your white nail has dried for SEVERAL hours, cut your tape pieces. I usually use scotch tape for straight lines, but I thought I'd try painter's tape again. It doesn't matter what the pieces look like, they just need to have a right corner on them .
|painter's tape strips|
Tape the white nail so that the top right and the bottom left are covered (see pictorial below) and paint over with the blue. Remove the tape right away! I did find that I didn't get as clean of a line as I wanted, so I took a tiny brush dipped in polish and made the edges a little straighter.
|From L: SH Black out, OPI Alpine Snow, OPI Suzi Says Feng Shui, Seche vite|