While I use the standard 30" circle of lexan for pattering, I also the 20" (of lexan) centre to show thickening which is necessary for long range killing patterns.
To pattern your pelter, mark an aiming point as noted by RB, aiming at it as you would hold on a sitting bird. Note where the centre concentration hits and the whole of the pattern at well. Find the centre of the pattern and see where your gun hits- then adjust the way you hold it to 'hit' the centre. Wonderful is the gun which shoots 'true' - that is, hits centre on your aiming point. This, of course, gives you the best opportunity for killing the game. It's bad if the gun shoots high, low or to one side or the other. Changing the load composition can sometimes move the zero around a bit, but sighting or bending the tube might be necessary.
Patterning can be done at close range, ie: 25 yards, but for bird shooting on the wing, 30 to 35 yards might be a better range. Any choking in the tube should allow patterning farther out.
Many ML's shoot low with shot - patterning is the only way to find out where the gun shoots. Shooting at water, ie: a pond, shows next to nothing worthwhile.
If you visit a 'Forestry' building, they may have large maps ready to be thrown out - these make good patterning paper, as do rolls of butcher paper.
Fowl's target shows wonderful central pattern concentration, but if the little round circle at the top of the 18" circle was the aiming point, it also shows the low hitting I mentioned above. One would have to practise in raising the eye above the breeches enough to centre the pattern, especially for 'flying shooting'.
Joseph Manton was the first to incorporate raised ribs to compensate the this 'low' shooting. Of course, the larger the breech diameter, the higher the impact of shot, but then, a ball from the same gun will tend to hit too high.