The first step: check to be sure that all the trim fits correctly. Ideally, this should be done before the paint, but at least do it now while it's still possible to make adjustments. with the trim in place, draw a pencil line along the bottom of the trim and then place masking tape about an eighth of an inch up from your line. The tape will mark where to cut the vinyl. The entire roof should be scuffed with a medium Scotch-Brite pad to cut the gloss and to help the contact cement get a good grip.