For the ones that I have done I have picked up a few bottles of the 30% stuff for a hair supplies store and mix it with some borax powder to make a paste. Then take a paint brush and brush down every part of the skull I can very liberally. Then I take what left of the paste and but it on some rags and wrap the skull in it and cover with Seran wrap and put it in the sun for a day. The sun will cause a little heat therefor making the hy peroxide work better. This is why you see ladies with there heads in heating lamps at salons, heat really activates it but do NOT put it near any other heat source, it is highly flammable, just in the sun will do. Give it about a day and peal off the rags and you'll have a perfectly white skull and never have to worry about the yellowing bleach may cause or bone deterioration.
Here is a picture of one, its not the best pic but it shows that it turns out VERY white. Good Luck!