I'd be glad to post pics except I recently introduced a new shooter to flintlocks and I gave him mine. They're easy to make. An old chainsaw file can be used, just break it to length, maybe 3" long. Using a bench grinder or a dremel make sure both ends are flat and square to the length, then carefully grind a shoulder on one end maybe 1/16"-1/8" smaller than the overall diameter of the tool. What you want is a small shoulder in the end so when placed at approx 90 degree angle to the top edge of the flint, you can strike the tool and knock a spall(small chip) off the bottom of the flint. The depth of the shoulder and the angle it is held controls the amount of flint knocked off. You don't want to take much off, just enough to produce a sharp edge. When you're finished, you'll end up with a saw toothed front edge to the flint. I like to even it up with the heel of the frizzen just a bit to level the cutting edge just in case there are some high spots, it keeps the frizzen from knocking off large pieces of flint when shooting, and even preventing the flint from fracturing. hth, Tim
Edit: This process is for an unloaded gun. I've done it with a loaded gun, but there is always a risk of a spark finding its way thru the touch hole. I've never seen a spark doing this, but it's possible and murphy will always make it likely. Brass works too, but doesn't last long, the flint being much harder, wears the shoulder off.