I posted pictures of the process of knapping the flint in the rifle lock some time ago, but here's a couple to refresh that idea. And these are two different hammers that I made of a piece of copper bar and a flat screw driver shank. Now I just make the screw driver from drill rod, forging the bit and hardening and drawing the temper. That way, I can make them to match the top faw screw's slot...some are very thin while others could be turned with a tomahawk.
The first is made from a cut off screw driver, with a 1/2" round section of copper bar 2" long. The second one is from a piece of copper from a big motor that is 1/2" wide and 3/8" thick. The flatter one is nicer in the bag for obvious reasons, and has lots of weight for knapping purposes.
When using the hammer, you strike the flint downward and into the flint at the angle I've shown. If you strike straight down, you get a short flake that only makes the flint blunter. The idea is to drive off a flake from the underside of the flint to thin the edge. I place my index finger under the flint's edge - the stone likes pressure from beneath, and produces thinner and longer flakes that way. And in doing so, I lift the cock off the half cock notch, using the weight of the rifle to hold it there while I strike the flint. That way, I'm not bashing down on the half cock notch and the sear's fragile end. When done this way, you shorten the flint very little, but thin it a lot. You can see the flake I've struck off laying on my finger.