Tape off your headlight, leaving the area you want to by "lidded" untaped. Adjust so that both sides are even, and have the shape you want. Take your time here. Once you are happy with the shape, spray a few coats of plasti dip on in quick succession (don't let them dry all the way between coats, ~5 mins drying time depending on conditions). After the 3rd or 4th coat, quickly and carefully remove the tape from the headlight. By removing the tape while the plasti dip is still wet, you will get a nice, clean line.