The way I do french tips that gives me a really clean line is like so:
1. wipe off all nail polish
2. put scotch tape on each of your nails, leaving room at the tips. Make sure its applied completley, so paint cant get under it
3. paint the tips of your nails with white nail polish or color of your choice, its okay if the paint gets on the tape.
4. let polish dry completely before removing tape.
5. apply a clear coat of polish and let dry.