once you get it sit on the car the way you want it. i take a nail head slide it under the top and hook it on the lip of the top of the car and tap it with a hammer on top. going around it till i can connect the dots with a stright edge. then take a magic marker mark the outside of the marks. then use a 4 in side grinder with a paddle wheel 80 grit to cut the final 1/4 inch. jonathan