It's fairly simple.
Make a patterning board out of a piece of paper applied to a cardboard or coreplast backer and set it about 20-25 from your shooting position.
Place a black mark in the centre of the paper.
Raise your gun to your regular pre-mounted position and look at the black mark in the middle of the paper.
Fire 5 shots at the paper using the bottom barrel.
Then repeat using the top barrel.
Look at the concentration of pellets relative to the mark on the paper.
Circle the densest concentration of each paper. Stack them one over the other and that's where your gun is shooting relative to you.
If you have an adjustable comb, move the comb in the direction you wish to move the pattern. Ed if you need to raise the pattern, raise the comb. If you need to move the pattern to the right, move the comb to the right.
If you don't have a comb, you can buy a Jones pad, or a Morgan pad. They have a sliding plate that allows you to move your whole stock relative to your recoil pad. You can also get just the sliding plate that fits between the recoil pad and the stock.
Midway USA and Brownell's both sell them.