Buck and ball was the standard load for smoothbore muskets in the U.S. Army, as well as the Confederate. The cartridges were formed by rolling the tube, twisting and tying off one end, then placing three .31 cal. round balls in the end of the tube and tying a half hitch between the balls and the end of the former. Next a .650 round ball was seated in the tube and another loop taken with the thread between the .65 ball and the former, and tied off with a full hitch. The former is withdrawn from the tube, and a charge of 110 grains of black powder added before the tail of the cartridge is folded and closed.
To load and shoot, tear of the tail of the cartridge and pour the powder down the bore, then place the buck & ball, paper, string and all, down the bore and ram it to where it's fully seated against the powder charge.
It doesn't really matter if you load the load with the .65 bal on top of or under the buckshot. Personal experimentation indicates that if you load with the round ball up you get a little tighter pattern than with the round ball on the bottom. And yes, the paper and string do ignite sometimes and fall a short distance from the muzzle, which is why under dry conditions the woods sometimes caught fire during Civil War engagements.
The same charge was used in both the .69 flintlocks and percussion smoothbores, a reason for the heavy charge of powder being that the flintlock was primed from the cartridge before loading the rest of the cartridge into the bore.
12 gauge shotguns, commonly used in the Confederate ranks in the first year or so of the War, were able to use the .69 caliber cartridges, which made ammunition issue a little simpler.
Tom