You measure the length of the main beam, plus the length of each point, plus the circumference between the head and 1st, 2nd, 3rd and 4th point. Add these all up, plus the inside spread and that gives you the gross score. Boone and Crockette is the most common scoring system, here is the link for a detailed description of how to score. You can do itself for a very rough score, but will need an official scorer to get a very accurate score.

http://www.boone-crockett.org/bgreco...Whitetail+Deer