This is the way I do it, their is a thick spot on the inside of the skull just under the spine hole, I drill a small hole so it don't split and put a big self tapping screw in it through the board, drill the hole through a little bigger than the screw at a slight angle where it will pull the skull in tight and flat, then after I get it tight I put a little supper glue around where the screw go into the skull in case it shrinks a little, never had a problem, also do it this way on deer heads.
Pappy