It's fairly easy to do yourself...even I can do it.
Remove all skin, meat, eyeballs, ears etc. from the head.
Sounds a little sick, but poke something like a straightened out coat hanger formed into a hook on the end in and out of the brain area to loosen it up. This is fairly easy to access thru the hole in back of the skull where the spine connects.
Boil the head in water (outdoors of course). Put a detergent in the water like Tide laundry, or Dawn dishwashing. Both of these are good grease cutters and will keep the grease suspended in the water and out of the skull. This step will take some time. Change water/detergent if it gets too yucky.
Remove the head periodically and zap it with a water hose to remove loosened tissue....scrape and gouge with a knife to loosen it up.
Once all tissue is gone, inside and outside, wrap the antlers in foil and tape. Mix a new batch of water with one quart of hydrogen peroxide...bring to a boil. Suspend the head in the water with the antlers out of the water. The peroxide in the water will bleach the antlers and change the color...don't want this to happen. The foil/tape keeps the rising fumes off the antlers.
The above step will bleach the skull and remove any remaining grease in the skull.
Rinse with clear water, let dry, and mount it on the board of your choice.