Make sure your rifle can shoot the kind of powder you select
I would add after the above words to the effect a flintlock ignition can only shoot real black powder IE: Goex, Swiss, Schuetzen. A flintlock is not made to shoot substitute powders such as Pyrodex, Triple 7, etc. A flintlock is also not designed to shoot powder in pellet form but only loose. Also loose powder comes in different granulations. 1F being the coarsest and 4F the finest. 4F is used in the pan and is never used as the primary charge powder.
The above is pretty well known to any flintlock shooter who as been doing it for a while. But for a beginner it is helpful stuff.