I installed a rear lip on my bumper.

there are paper templates on where to DRILL HOLES. Be really careful, triple check everything, start with a small bit, and work your way up.

You can STOP short of the 8 or 10mm hole needed. It'll just make it tighter.

When drillling, use a fast speed dremmel to start a pilot hole. If you go slow, you might spiderweb the paint.