Flintlocks are meant to be loaded previous to combat and used as a quick first shot (they fire in one second) before resorting to another weapon, though they are somewhat effective when used as a primary ranged weapon, if desired. They are a bit weaker than their other ranged counterparts and require more effort and money to maintain.
Flintlocks require both shot and powder. Both can be purchased in the previously-mentioned store. Shot can be bundled. Once you have both shot and powder, simply LOAD the firearm twice - first for the powder, second for the shot. The shot and powder simply need to be inside a container you are wearing - you don't need to manually get them out first and then LOAD the flintlock. Example:
You can LOOK at the flintlock to see whether it is loaded with powder or shot:put powd in bag
You put a belt powderhorn in the weathered canvas duffle bag that you are wearing.
put shot in bag
You put a bundle of shots in the weathered canvas duffle bag that you are wearing.
You remove a steel-barreled flintlock from the weathered canvas duffle bag you are wearing.
You retrieve a shoulder powderhorn from your weathered canvas duffle bag and carefully load some powder from it into your steel-barreled flintlock, then stow the shoulder powderhorn.
Roundtime: 3 seconds.
You retrieve a lead shot from your weathered canvas duffle bag and load it into your steel-barreled flintlock.
You pull the trigger of your steel-barreled flintlock.
You fire a lead shot at an animated training dummy!
Ranged: 108 vs Dodge: 100
Roundtime: 1 second.
You see nothing particularly unusual about the steel-barreled flintlock.
It is loaded with powder.
It is loaded with a lead shot.