I once put a bear skull in a 5 gal bucket with a bunch of holes drilled in the lid and tied it to the bottom of a tree
beatles do a awesome job and they get everything and you don't have to do anything other than put the skull in and close the lid
I've tried the peroxide that is recommended by everyone after buried and it doesn't seem to work. I had them in a garage for a few weeks lathered in the stuff. Can't remember name, but got at a salon product store.
Key word there was "garage". You need to have the skulls in the sun. The combination of the sun's rays and the peroxide will whiten them right up. I just use the peroxide used for cleaning wounds. Don't go buying anything expensive.