Well each Bess will act a bit different, but...,
Often I have found that a properly sized, paper cartridge with a ball works well. I like a .690 ball myself and good, book quality paper. Some guys like a .715 ball and newsprint. 70-80 grains of 2Fg is fine. The cartridge should fit into the bore with ball and not bind, but not be so loose as to wiggle either.
Now to sight in a Bess, or other smoothie I have found, is you look at the front sight post. In the case of the Bess it's the bayonet lug, BUT we do have records of soldiers armed with the Bess being issued ammunition for "shooting at marks" so we know in some regiments that they did target practice with them.
Where was I...?
Oh yes..., look at the frong sight post as you shoulder your Bess. Now, you want to align the wood and tang portion of the stock, where the barrel ends and the tang continues, that flat area on top..., you want to align that flat area with the base of the front sight post. So all you should see is the very top edge of the muzzle where the front sight post is attached, and the front sight post extending upward from that point...
That gives you the elevation as you don't have a rear sight. Then simply place the top of the front sight post on the bottom of the black of the bullseye, and squeeze while concentrating on maintaining that sight picture. The front sight touching the bottom of the black on your target circle is sometimes called a "lolipop hold".
Start from there at 25 yards, then if you get a decent group, adjust your point of aim to center that group, and extend the target to 50 yards.
BTW I got the sight picture information from Muzzleloader magazine, and I believe it was an article by Mike Nesbit..., who shoots NWTG's a whole lot, and is pretty good with them.
LD