Lambo_Sam is right, its a pretty simple task. The most important thing is to use good SHARP tools when doing it, it will spare you the problem of chipping around the cut and more touch-up afterwards.
I have actually done this on 2 other already painted cars with no problems. The best way to do it is to make a pattern to route the hole into the fender with a top bearing bottom bit in a router. Make the pattern so it clamps onto the fender, and don't rush the routing. You will be left with a perfectly matching hole for the light, and a clean straight cut in the fiberglass that will be near "paint ready". all you'll have to do then is lightly sand the cut edge to give it a slight radius edge, then touch up the paint inside the hole.
