Then You need to put the skull in water in a plastic bucket, and carried far from the house and just let it rot in the water, do not submerge the horns.

It will stink but you will get the job done. Make sure you cover it with burlap or some other material and then tie it to a tree or something ,so that critters wont eat it or steal it.

Then you can wash it of with a degreaser and put hydrogen peroxide on it and place it in the sun until it is bleach white.