Um, on a pattern board, how else? Though I could have written more clearly, as the patterns aren't so much "messed up" as "moved" in most cases. I find them "messed up" because it makes it harder to hit with them when using shot.
I patterned my current 11 gauge gun prior to filing the muzzles to regulate POI for ball. My shot load consisted of 4 1/2 drams of Fg, 2 card wads, #4 shot equal in volume to the powder, 1 overshot wad. With the bead centered in a 30" circle at 30 yards, about 65% of the shot stayed in the 30" circle (pattern from the left barrel was a bit low, right barrel was a bit high, but both were centered on the vertical axis).
After filing the muzzles to regulate POI of a ball load at 50 yards, I again patterned the gun in the same manner at the same range. Pattern from the left barrel had moved up and hard right, and was now appx 30% in the circle, most of it hitting outside the circle at roughly 2 o'clock. Pattern from the right barrel had moved slightly down and hard left, and was about 50% in the circle centered on the edge of the circle at 9 o'clock. It is no longer a gun I would chose to take birding if I had an option.