Once you have the hole to place the lens in, you need to gring the back of the fiberglass so its uniformly about 3/16" thich all around the hole opening.
One one edge or the lamp, there is a metal spring tab the slips into a slot that you'll have to create on the backside of the fender for it to mount into. On the other side of the lens is a tab that works much the same way. If you want the lenses removable, you'll need to make the mounts. I've seen some people that just lue the lens into place on the backside with silicone. It works, and will hold it in place, but you'd have to cut the silicone away if you ever wanted to replace the lens.
