You need a target large enough to draw a 30-inch circle on it. Place a dot in the center of the circle for your point of aim. You’ll want to shoot the target at 25 yards when patterning skeet and cylinder chokes (and all .410 gauge chokes), and at 40 yards for all other chokes. This is the industry standard. Some people advocate shooting from a bench to minimize input from the shooter. If you do this, take pains to make sure your head is aligned with the stock the same way it is when shooting offhand, or it will change your point of impact. I prefer to shoot from a standing position by mounting the gun with the barrels pointed slightly upward and then slowly dropping the muzzle until it is aligned with the aiming point and firing. For best results you’re going to want to do this five times (at least) with each choke and load combination. It’s a time-consuming process, but there are no shortcuts here. Your targets will reveal pattern density—measured as the percentage of pellets in the shell that impact within the circle; point of impact; and pattern distribution, which hopefully doesn’t indicate any major gaps where a target can escape unscathed. If you have a double-barreled shotgun, you’re going to want to pattern with each barrel. In addition to the data gathered above, you’ll figure out if your barrels converge on the same spot. If you encounter any problems, the best course of action is to start by changing chokes and loads. If that doesn’t fix issues with where your shotgun is impacting or the quality of the patterns then you need to have the gun looked at to figure out what’s going on.