only submerge the skull to a point below the antlers. wrap the base underneath the antlers with PLAIN paper towel. Lay similar strips of paper towel across the top of the skull to cover it as needed. Then carefully pour your whitening liquid onto the paper towels to wet them. From there on out they will wick the fluid up and whiten just as well. If you use a high strength peroxide, be careful as it will foam and still climb higher than you want. practice makes perfect!
If by chance you get some on the bases of the antlers, there are ways to fix that.