Have you bought a GSI spoiler? If it's not a fibreglass aftermarket spoiler, there should be some metal bits sticking out the back of the spoiler (these should be threaded so a bolt can be twisted onto the end). What you'll need to do is mark carefully where holes need to be drilled into the tailgate. Drill some holes with the right size drill & slot the spoiler in the holes (there should be bolts to twist on the ends). I used double sided tape too to make sure the whole thing held down onto the tailgate well.