They are a pain in the rear end! I've used a lag bolt before up in to the antler then drill out a couple smaller holes in antlers and skull plate to put screws in or something to keep it from spinning. Epoxy or use fiberglass resin to glue the lag bolt and screws in once you figure out a way to position them and keep them in that position until everything dries.
A lot of times the skull plate isn't anywhere close to spread of the antlers so you have to modify it to get a good look for you particular set.
Hope this helps and makes some sort of since.
Any questions fill free to call. (425) 433-0231
Rick