It's very easy to do yourself, and there are several different methods you can use.
Just clean off as much as you can, start by skinning, remove as much meat/flesh as you can.
Some guy's like to burry them and let nature take it's course....others like to sumerge them in water until it all rots off.....some like to simmer them in a pot of water mixed with dish soap, and or borax to help break down the fatty tissues. You don't want to boil the skull....or use bleach.
If you simmer the skull....and pick away all the meat etc as you go....once it's clean, just pick up some peroxide paste from a local beauty supply shop, and leave that on a couple hours and rinse off. Whalla...you're done.
If you do a search, there is a ton of great info on this site.
Ben