Get 40% ( by volume ) peroxide from a beauty supplier like Sallies Beauty supply. Mix it with Whiting, that you can get from Van Dykes or McKenzie Taxidermy Supply. Make a paste and, using a paint brush, cover the entire skull with the paste, wrap it in saran wrap ( to keep the paste from drying out ) and put it in the sun or under a really bright light or a UV light such as a tanning bed. After a couple of days, clean it off and repeat if necessary.