Ok, buddy, I was a professional taxidermist for 20 years so maybe I can help. First, if you just lopped off the head and did nothing to it, you probably have a smelly mess like you described - depending on how long you've had it and how warm the temps have been. If you've done nothing to it by now, it would be best to just get a saw and cut off the top of the skull with the antlers attached and clean it up the best you can.
In the future, if you want to do a European mount (skull), you should always start by skinning the skull and removing the lower jaw. Then cut as much meat away from the skull as possible. Also remove the eye balls from the skull and tongue from the jaw if you plan to kjeep the jaw. Next, get some sal soda (it's also called soda ash) from a taxidermy supply house or you might be able to get it at a pharmacy. Put about 1/2 cup of the powder into a pot that's big enough to immerse the skull and get it boiling. Do this outdoors if you can 'cause it's going to smell. Every 5 miuntes take the skull out and try to scape the remainig flesh off the bone. It will get like jelly eventually, and come right off. It will get so soft that you can use the pressure of water from a garden hose to get most of the flesh off. You have to check it every few minutes because you don't want to over boil it as the bone itelf will get too soft. Then get a wire coat hanger and stick it into the hole at the base of the skull, and keep twisting it around until you get ALL of the brain matter out. Wash the whole thing in a bucket with a TINY bit of dish detergent to get any fat or grease off the bone, and rinse it off really well. Good luck.