First thing first, componants....I've loaded plastic and paper hulls but never brass with black powder. The principle remains the same no matter what type of hull one uses. You have to have a primed hull, powder, a way to seal the powder off from the rest of the componants, some kind of cushion to protect the shot, and some way to retain all these things inside the hull. The method I use most is the old card & cushion wad system. With this type of loading one measures powder into a primed empty hull, followed by a heavy card wad that is usually called an over powder wad, then a 1/2" cushion wad is added followed by shot, and the whole thing is crimped in a normal manner with a star crimp.
The other most common method is to use modern componants instead of the card and cushion wads. Most folks find the Winchester red wads are pretty good for loading over black powder. Pretty much the same scenerio, load a primed hull with black powder, seat a plastic wad, add shot, and crimp. The plastic wad acts both as a gas seal and a cushion. These will work fine in most guns! However they will leave copious amonts of melted plastic in your bore. No big deal though, at clean up time simply squirt windex down the tubes, let it set for a few minutes, and then push a heavy paper towel or wad of news print down the bore. The plastic will come out in a stringy mess, but it will come out!
Other aspects: I have found that older scatterguns, say pre 1960s, tend to work quite well with the tradtional card & cushion wads system. They have very short forcing cones, which is what these type of wads were designed for originally. Later guns tend to have longer forcing cones and sometimes are even back bored. This type of gun is intended for modern shot shells using plastic wads and they don't usually pattern very good when loaded with the old card & cushion wads. This seems to be because the longer forcing cones allow powder gasses to get into the shot column as it passes through the long forcing cone. Combine that with the lack of chokes in most "coach guns" and you have disasterous patterns. I have found that a combination of technology is the best solution in these guns, using a heavy over powder card wad in combination with a plastic shot cup with the cushion cut off! This keeps the gasses from disrupting the shot charge and also cuts down on the plastic fouling building up in the bore!
Roll crimping: I don't do roll crimps, but they are a very authentic method of loading paper hulls, and can even be done to plastic hulls. This utilizes a thin card wad on top of the shot and the sides of the shell are "rolled" down to secure it. Orginal roll crimping tools are often seen for sale at gun shows and on e-bay. If you are into that sort of thing go for it! They are cool looking when folks can see ya popping them into the chambers, and they work just as well as any star crimp!
Brass hulls: As previously noted I have never messed with brass hulls. The Cool Factor with them is MAJOR! But the expense has always put me off my stride when it comes to investing in them. I HATE the idea of stepping on one and ruining it during a stage! Nevertheless lots of pards use them, for the cool factor if nothing else. Loading them is not much differnt than loading any other hull, powder, then wads, then shot, topped off with a thin overshot wad glued in the top of the hull. Some pards use authentic watergless to seal the overshot wad, but hot glue seems just about as prevelant as anything, with plain old Elmers white glue near the top of ythe list too.
Loads: The ancient rule of thumb is to load with the same volume of shot as powder. Ergo a measure that throws a given amount of shot such as 1 1/8 oz is the same volume that you want to use to throw your powder charges with. This is not a hard fast rule but is always a good starting point. I've noticed that if I want to make changes it is best to go in the direction of more shot than powder, in order to maintain a good pattern. When I have tried it the other way, more powder than shot, I always end up with really crappy patterns. I can't explain it, but that's the way it works.
Finally: You are going to have to experiment with your loads and gun to find what works best for you. I started loading black powder in an ancient Crescent 12 bore and got fabulous patterns with the old card & cushion wads. This gun was designed for them and its chokes throw thier very best patterns with them. Your gun is different than mine and what works in mine may or may not work for you, particularly in an unchoked gun. But the above information should get you started in the right direction to find happiness with black powder and scattergunz!
Pards, please feel free to add information that you have discovered! This is a topic that needs all the input we can get!