This FAQ answer from Dixie Gun Works:
The proper sequence for loading a muzzleloading shotgun is as follows:
black powder
over-powder (heavy card) wad
fiber cushion wad
optional shot (thin card) wad
shot
over-shot wad
Recommended loads are as follows: 20 gauge -- 65 grains of FFG black powder and 1 ounce of shot, 12 gauge -- 75 grains of FFG black powder and 1 1/8 ounces of shot, and 10 gauge -- 85 grains of FFG black powder and 1 1/4 ounces of shot. Dixie stocks all of the wads needed in assorted gauges. Muzzleloading shotguns can be loaded with patched round balls as well as shot. We recommend using a .600 round ball for the 20 gauge, a .715 round ball for the 12 gauge and a .760 round ball for the 10 gauge.