I'd recommend the following load to start;
+- 80 gr 2F
over powder card
cushion wad
1 to 1 1/4 oz of suitable shot
over shot card
After you try that you can modify it to suit. A traditional load is equal VOLUME of powder & shot. To make it a true "square load" the shot column should be the same HEIGHT and DIAMETER. Don't bother to work it out, it merely approximates a light load. According to the English shotgun writer Gough Thomas, the best patterns come with an ounce and a 1/16th of shot. The weight of shot should approximate 1/96th of the gun weight. That is one ounce for each 6 pounds of gun. Less powder, or more shot, can be used, especially if you want a tighter pattern. I have used LEE .31 cast roundballs as buckshot in my CVA ML shotgun. It put all 12 in a one foot circle at 25 yards. There were no tricks used in loading them.
Here's a link to an old thread about making paper cartridges. For ball, get a
.690 mould and load it like a patched ML rifle. Same load to start, about 80 gr 2F.
http://www.gboreloaded.com/forums/index.php/topic,89944.msg554623.html#msg554623