The easiest way to knapp your flint is to use the frizzen on your gun to do it!
With the gun unloaded, and unprimed, cradle the gun in your left arm. Use the thumb of your left hand, the outside edge. to lift up the frizzen while you manually lower the cock with the flint mounted in the jaws, until the bottom corner( heel, if you compare the "L-shaped Frizzen " to the human leg and foot)of the face of the frizzen.
Hold the frizzen in that position with that left thumb, making sure to keep your thumb back from the face of the frizzen so you don't slice off a piece of the thumb doing this.
Then, cock the hammer back to full-cock. and pull the trigger. The flint will hit very low on the frizzen, where the frizzen corner is strongest, and at a sever angle, so that a long spawl is knocked off the bottom of the flint edge, giving you a new, sharp, even edge all across the width of the frizzen.
This is the fastest way to knapp a new edge on a dulling flint.
Before going back to shooting, however, hold the gun out in front of you, at eye level, and cock the hammer back to fullcock, lower the frizzen, and looking at the pan, fire the gun by tripping the trigger with your left thumb. You want to see where the sparks are landing in the pan. If they are landing to the rear of the pan, or even behind it, or on the fence, or God Forbid, BEHIND the fence, then you need to release the jaws, and move the flint forward. Hold it in that new location by using a twig or stick to wedge behind it, between the back of the wrap, and the cock screw. If the flint is too short to be held firmly by the jaws of the cock, its time to change the flint.
In an emergency, you can turn a short flint (stub)sideways, and get a few strikes out of a new edge made from the former "side" of the flint.
When the flint is moved, or a new flint is installed, its next to impossible to get the flint mounted in the jaws exactly parallel to the face of the frizzen. The edge may not even be square to the length of the flint. So, mount the flint as square to the face as is possible, and then knapp it as described above before shooting it.
Always check to see where the sparks are landing from that new edge. It will give you an idea on how many strikes you are likely to get from that flint before the sparks stop igniting the powder.
Ideally, with a new flint, you want the sparks to be hitting forward of the center of the pan, so that as the flint wears down, the sparks just slowly move rearward, first to the center of the pan, and then to the rear of the pan. How much priming powder you use in the pan will be determined in part by where the sparks are landing, NO?( What's the point of throwing sparks if they don't hit powder???)