Before you do anything you need to determine what chokes you have on the barrels at present. Fixed choke guns typically have the choking for each tube stamped at the rear of the tubes. Some brands will use stars number from 1 to 5 starts and some brands will use a 3letter code such as CYL, IC, MOD, IM, FULL. or some variant of this to designate Cylinder, Improved Cylinder, Modified, Improved Modified, and Full.
I shoot 4 gun Skeet and and used up 11,000 primers last year. Most of my practicing is done with the 410 because it's the least expensive to reload and because it enforces shooting precisely. One thing I've learned from the 410 is that you only need 1/2 ounce of shot to break the targets. So if you are shooting a 12 gauge what your barrels are choked to is pretty irrelevant, put your shot string in the correct place and you will break that target. What matters more than anything else is how your gun Fits you. Because for skeet the shotgun isn't aimed at a target, it is actually Point Shot. As in you look at the target and when the sight picture feels correct you pull the trigger. In order to do this well you need a gun that "shoots where you look", which is where Fit becomes so important. One Polish Engineered approach is to use black electricians tape to completely cover any sight beads on the barrel. Then you take your gun to a patterning board, Mount the gun, look at a reference mark on the pattern target, and pull the trigger. If the gun Fits well you'll find you have a well centered pattern.
Corrections for non centered patterns.
First is left to right. The rear sight for a shotgun is your eye positioned along the line of the rib. If you pattern fall to the right of the target this means you need to move your eye to the Right. If the pattern fall to the Left you need to move your eye to the Left. Now an interesting aspect of Human Anatomy is that our faces are are shaped like a triangle when viewed from above at the point where our face contacts the butt stock. We can use this to adjust the position of our shooting eye relative to the centerline of the rib. To keep it simple I will go thru how to adjust your head rotation to shift the eye position for a Right Handed person. If you want to move your eye to the right take your left index finger and push your nose closer to the stock. If you want to move your eye to the left then pull that nose away from the stock. Once you have your eye perfectly centered on the rib you will be able to see that visually. At this point take note of the line of pressure and how it presses your cheek to your upper teeth. You want to memorize this this line of pressure so you can use this as indication that you have mounted the gun properly.
Now for Up and Down. This can get to be expensive, because your eye position vertically is determined by the comb height of your gun stock. Note, within manufactureres there can be variation in this height up to 1/16 inch from stock to stock of identical models. IF you find your patterns are a bit low this means you have to move the eye up. In this case you can purchase a self sticking gel pad made by Beretta from Midwest Gun Warehouse. There are two thicknesses available, IIRC it's either 1/8 inch or 1/4 inch. If you need a higher comb this is a good place to start. Because the alternative is having your stock modified into an Adjustable Comb Stock which was about 400 dollars back in 2020, in today's inflated world it's probably a 600 dollar job. If you find the gun is shooting high this means that you need a lower comb height. In this case one option is to see if lowering the rear of the stock at your shoulder will get your pattern centered. Unfortunately doing this can cause unpredictable results in the cheek weld but some do get lucky. If you find that doesn't work then the next step is to lower the comb on your stock, which means get out the belt sander. Actually this is something you'll want to have done by a professional because a full refinish of the stock will be needed after it's been fitted to your face.
