My buddies and I have been doing our own for ever. Working Saturdays in a butchers shop as a kid provided me the basics. Sometimes work or weather drove us to use a commercial butcher but the results have disapointed.
A really good guide is :
Game: The Art of Preparation and Cooking Game and Game Fowl
by Klaus Wockinger, Douglas Leighton
Softcover, N & C Publishing, ISBN 0969365322 (0-9693653-2-2)
Hardcover, N & C Publishing, ISBN 0969365314 (0-9693653-1-4)
Wockingers recipe for bratwurst is the best I've found - he is a hunter and a chef.