They can be a pain. I used some fishing line (some say dental floss works as well). The outer parameter of this cap has some real good two sided tape to adhere it to the rear deck. There are also alighnment pins in its center, so work your way around the parameter with the fishing line.
You will need to clean off the tape from the deck and faux cap, then reapply new tape to stick it back on.
Because of this, some simply mask off the back end of the car, and paint it while still installed. Mask well, as you don't want overspray to settle anywhere on the cars surface!
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.