i am worryed useing a knife to cut it but what is the best way to cut it without messing up the paint
I used an exacto knife/hobby knife. Trick is not to cut all the way through and just peel away the excess. Cut out a piece of material that will be slightly larger than the grill, then use the straight-edge of it on the top portion. Just take your time, go very slow, and use lots of water and squeegee. You will do fine.