I'm not sure of the exact process, but what I've seen alot of guys do is make a hole in the bottom of the skull big enough for a small piece of wood. They secure the wood to the skull w/ bondo or something similar, then paint the piece of wood white so it doesnt stick out. Then you can screw the piece of wood to the plaque. If you need some more help pm me and I could suggest another site to check out. Not sure if I could post it here w/o violating forum rules.