Answer A.) Cut one side first. Then, slide it over about ¼ of an inch on the other side for a clean install.
Answer B.) First, you need to go in and trim the inside gasket and cut the wing. Then, you need to thoroughly clean the area and let it dry before applying a light amount of water when installing. Tape or cover the dirty areas of the truck and make slightly larger cuts than you would normally to compensate.
Answer C.) Trace out the pattern on the film with a silver sharpie. Then, hand-cut it on a peel board.
Answer D.) Inform your customer that the old gaskets will probably be destroyed when you take them out. Let them know that replacements are cheap and that they should order new ones. Also, let them know that you’re not responsible when the gaskets inevitably break apart during the installation process.