Case prep is the same for a black powder cartridge, as for a smokeless cartridge. Prime with the same primers you would use if the cartridge were to be loaded with smokeless powder. Once the cases are cleaned, sized, and primed, they are ready to be charged with black powder.
When loading with black powder you must NEVER leave an air gap between the powder and projectile. Use a drop tube of about 24” length with a funnel on the top. The tube should fit just inside the cartridges mouth. You pour the powder down the tube into the cartridge. This allows proper settling of the powder charge.
Black powder cartridges were designed to hold the proper amount of black powder and the case will always be full, double charging is impossible. Normally black powder is compressed a bit when the bullet is seated.
So you subtract the seating depth of your bullet plus any wads you may be using from the length of the case, add in the amount of compression you plan on using, and this will give you the level to which you should fill the case with black powder.
Next you must add wads, to separate powder from black powder bullets and lube, this also increases accuracy potential as well.
Black Powder cast bullets should have large grease grooves to hold ample amounts of lube. Cast bullets for smokeless powder contains neither the right type of lubricant nor enough lubricant, so you will need to use cast bullets specifically designed for black powder.
Now that you have the proper bullet loaded with a good black powder lube seat the bullet and crimp in place. In revolvers and lever guns where the cartridge will be subjected to the shock of recoil a heavy crimp is needed.