All of the basics of safe firearm handling that apply to modern guns apply to muzzleloading firearms as well. Never use any type of modern, smokeless powder.
Black powder has a totally different chemical formula than smokeless.
Always seat the projectile directly onto the powder charge, never leave a bullet part-way down the bore. If you fire many shots without cleaning the bore in between, you may reach a point where the bore is so heavily fouled that you can't seat the next round. If a bullet should become stuck party-way down the bore, don't try to shoot it out as it could burst, or at least bulge, the barrel. If necessary, drive the bullet down with a heavy rod and a hammer, then fire it. Failing this, pour several tablespoons of solvent down the bore.