This looks like the template for cutting the openings to the underside of the deck lid. Recall, the decklid is two (maybe more) stamped pieces of sheet metal. Some decklids come with these already stamped out. Others have to roll (cut) their own. Once cut, you then can access the nuts/fasteners to hold the spoiler in place. Of course, AFTER the holes are drilled through the top of the deck lid.
Cutting through an already nicely painted decklid creates lots of anxiety for me.