AFter the meat has entirely sloughed off (you may need to loosely brush some of it) soak it in a solution of Dawn dish soap water for a few weeks. It will degrease it. Remove all remnants of meat, fat etc. , I use an air hose to blow out the cavities (carefull, the teeth will go flying and bacteria laden glop will blow all around). When the skull is effectively clean, dry a day or so then soak in a solution of hydrogen peroxide to kill the bacteria and bleach it. It doesnt have to be all that strong- a quart of over the counter stuff from kmart in enough water to cover the skull is good enough, for a day or two. Then thoroughly rinse it to remove the peroxide. AFter drying, spray with acrylic clear spray.