Patterning a shotgun is an important step in getting the most out of your gun/choke/load combination. It can be time consuming, but it is well worth it for those who are serious about performance from their firearms.
The basic way to pattern a shotgun is to shoot at a large target (usually more than 30 inches wide) at the distances you will be hunting. This allows you to determine how wide and dense your pattern is at those distances, allowing you to fine tune it for maximum efficiency.
To start the process, you will need a large sheet of paper, preferably one that is at least 36 inches wide, and a sturdy backstop. You can purchase rolls of brown contractor paper at most hardware stores. Make sure to staple the paper firmly to the backstop, so it cannot be moved during the testing process. A good sharpie is also a must to mark the center of the target and write any notes that you may need.
The next step is to fire several rounds at the target. When shooting, you should try to get a few shots within the same area of the target, allowing you to better determine how consistent your pattern is. You should also count the number of pellets that fall within a designated circle on the target. Various tables provide nominal pellet counts for different shot weights, but it is best to open three or more shells and count each individually.