I'd suggest you start on Youtube.
Seem everything ranging from some very well executed copies of the Littleton shotmaker, all the way down to a Russian dude making quite serviceable shot with a cat food can with a bunch of holes punched in the bottom with an old nail.
I'd next suggest some quality reading time over on the Castboolits forum, they have a specific forum for casting for shotguns, and the subject of shotmakers is usually got at least a couple threads running.
