If you don't want to buy strips made for French manicures, just use Scotch tape. Place tape exactly where you want the nail polish line to be. If you mess up and have to get it straight, you still have plenty of tape.
Or, you can just paint the tips of your finger/toe nails with white. Then take a small flat edged brush, dip it in acetone, remove excess and lightly move brush back and forth where you went out of your lines.