Depending on which bumper you have. I have the ones made by Token and there was a molding dimple of the square holes for the plastic license plate stand offs. A dremel made quick work of the fiberglass. After the holes are made, go buy the license plate standoffs. Maybe you buy the stand offs first then make the holes.
I just used the stock license plate bracket and mounted it onto the fibereglass bumper with two screws under the bumper the same way it's attached to the stock steel bumper...