Two questions, can I determine if my gun fits me well without help, and how do I pattern my gun?
Checking gun fit is best done with another person - preferably someone who knows gun fit. A common method is to mount the gun as if you are about to take a shot (before beginning this process, proof the gun and make absolutely certain it's not loaded. Then, get the other person to check it, too. This is important, because you'll be pointing the gun at his/her head.) As you stand there with the gun mounted, the other person looks at your head, neck, arm, and shoulder positioning to make sure you aren't twisting yourself to accommodate the gun dimensions. If all looks good, he/she then stands directly in front of the muzzle of the barrel and eyeballs straight down the top rib. Looking flat down the rib, they should see about half of your pupil. In other words, your eye should be in the same position as theirs, only at the opposite end of the barrel.
Another indicator of gun fit is to mount the gun with your eyes closed. After mounting the gun, open your eyes. If the gun fits you well, you should be looking right down the rib.
Patterning a shotgun is simple. Buy and bring with you a good selection of different brands of shells. If your range has a patterning board you can use that. If not, you'll need to set up a backboard roughly 3 feet square. Some folks use metal backboards and spread a thin layer of grease on it. After each shot they look at the results and then re-spread the grease for the next shot.
I prefer a wooden backboard with large sheets of paper fixed in front (Flip chart paper works well. One at a time - not the whole pad at once). I use a black marker to put a 1" dot in the center of the page. Start 16 to 20 yards back, aim at the dot like you are trying to hit it dead center with a rifle. Fire the shot, confirm the gun is safe, and go to the board to see the results. You want to observe:
- how much of the pattern is above center and how much below center?
- how much is left of center and how much right?
- how evenly spread out are the pellets. Are there any gaps large enough for a target to pass through?
- which shells generate the best results?
Make sure you're using the choke that you would use at the distance being tested. It's not unusual for a field gun to shoot 60/40 (60% of the pellets above center and 40% below). This is because wild birds tend to rise quickly when they bust cover and more misses occur because people are shooting below the bird, not above. Competition guns should shoot flat.
Once you've tested all the various brands and loads that you brought with you. Back up another 5 yards and do it all again. You will find that certain shells are best in certain guns. Also, different loads and shot sizes will produce better results at various distances.