The loading procedure for a muzzle-loading hand
firearm (as opposed to the procedure for a muzzle-loading
cannon, which is different in several ways) is as follows:
Hold the weapon upright. This is intuitive, but it bears mentioning because that is one of the most significant disadvantages, since the gunner had to stand in order to load a muzzle-loading rifle (pistols are of course different.) This presents them to the enemy rather well.There were a few breech loading flintlocks made, but they were expensive to make due to the high degree of fit-and-finish required to properly seal the breech from the powder explosion.
Pour in your powder. This was done at first by eye, the powder carried in bulk in that icon made famous by Daniel Boone, the powder horn. In some European countries, the powder was kept in individual vials to instill a level of consistency in the powder charges. However, many little waterproof bottles are more expensive than one big one. By the time of the American Civil War, the powder and bullet were pre-packaged in paper sealed in something like wax paper, but using animal fat. The soldier tore the paper open with their teeth to open the powder charge. (On a side note, there is a story that some black muslim soldiers refused to use the bullets as a rumour circulated that the paper was impregnated with pig fat, which they couldn't touch or put in their mouths.)
Insert a patch. The patch was a piece of cloth that increased the gun's efficiency by letting less of the explosion blow past the bullet. The cloth was eventually replaced by the paper used to wrap the round.
Insert Bullet. Once the round was packaged as a single unit with paper, the bullet was usually just rammed in with the paper in one go.
Charge the firing mechanism. In a matchlock, wheel lock, or flintlock, there was a small pan outside of the touch hole that led to the powder inside the weapon. This pan had to be filled with powder. Once the percussion cap was invented, a nipple to receive the cap replaced the pan.
Cock the firing mechanism and fire. Just don't go off half-cocked.
If you did all of this properly, the weapon would fire. Sometimes it wouldn't. If you were a novice, or were distracted in the heat of battle, you might reload a round right on top of your bad load. During the American Civil War, some of the dead were found with five or more bullets rammed down the barrel.