Momaw, you can try to remove most of it with a sharp knife and then place the bones in a bucket of warm water in a somewhat warm location if you can(it will get a little smelly). Ever so often pour the greasy water out and replace with fresh warm water. Once the water runs clear the bacteria has done it's work and you then soak the bones in regular Hydrogen Peroxide until it reaches the level of whiteness you desire.

Another option is to buy certain species of ants or beetles from Science Supply houses and let them do the work.

DO NOT BOIL OR BLEACH the bones. This is commonly recommended by people but it isn't a good thing to do. It cause the fat to soak into the bone making it greasy and off colored.