There is no one precise answer to replacing flints as each gun has a bit of "personality" & this results in exceptions to many of the "rules". That said, you should start with a proper sized flint. The flint should be about the same width as the frizzen (alternately called the battery or steel). You want to make sure that the flint is not so wide as to hit the barrel or edge of the pan which would damage both the metal & the flint. For most guns, the flint works best with the bevel of the flint in the "up" position. A few guns prefer the bevel down but if in any doubt, I would start in the "bevel up" position & only try down if either getting poor sparking or rapidly using up the flint. With the lock at half cock and the frizzen fully shut, set the flint in the jaws so that the sharp edge almost but not quite touches the frizzen. If in doubt for spacing, you could use a matchbook cover as a gauge. This distance seems to works well for most guns. A few seem to like the flint touching but as with the bevel, try starting with a small gap. Check to make sure that the flint does not touch the pan or barrel when the lock is in the "fired" position. If your flint is too long to fit all the way back in the jaws and not push against the frizzen at half cock (which results in opening the pan a bit which can lead to losing prime at worst and admitting moisture to the prime at best), you need to get shorter flints. Most guns use rectangular flints but a few like shorter square flints. If the flint is just barely too long, you can generally cut a hole in your leather or lead flint wrap to let it go all the way back to touch the jaw screw. Sometimes you can flake a bit off the back of the flint to get by but easier to just get proper sized flints. Tighten the jaws down firmly on the flint in it's wrap & work the lock to check for good sparking. After a few shots, check to see if the jaws need any retightening. The whole process of changing a flint is actually easier than this post makes it sound but I have tried to cover most possibilities. Good shooting.