I always use automotive masking tape. For a REALLY sharp, clean line,
cut strips of tape with a new #11 blade using a metal straightedge as a guide.
To prevent paint from bleeding under the tape, burnish it down real well or,
spray some clear along the tape edges and let it dry before shooting your color coat. There's bound to be different and possibly better ways to get the results you want, This is just one of the many.
Ray