I'm sure there are many ways to knapp an english flint, but I'll describe how I was taught.

As a right handed person, with a flint in the jaws, lay the rifle down across a padded table / bench and stand behind it with it pointed downrange / away from yourself;

Ensure the pan is empty, touchhole is plugged (or covered with a rag draped across it), and frizzen fully open / forward;

Bring the hammer back to full cock, then slide the forefinger of your left hand under the bottom jaw of the hammer to hold the hammer in place;

Then use a brass rod the size of a pencil, or the back of a hunting knife, and very gently just 'tap-tap-tap-tap' down on the leading edge of the flint at a 45 degree angle...left to right across the full width of the flint;

It only takes 4-5 very light taps so that tiny little flakes come off OF THE BOTTOM SIDE of the flint's edge...I think most of us when we start out think large visible chunks have to come off, but actually, all we need to do it barely freshen the leading edge...by flaking off a few tiny slivers, the leading edge gets thinner again, hence sharper.

Another thing that usually works with english flints IF THEY HAVE FLAT TOPs (not big thick high tops) is to simply loosen the jaw screw, and flip the flint upside down about every 10 shots...they will usually "self-knapp" so you can keep right on shooting...I usually average 40-60 shots per 3/4" black english flints that I've bought from October Country.

Remember, knapping a flint is just to eliminate the flat dents that buildup on the sharp leading edge after hitting the frizzen several times...just be gentle and don't expect to see large alterations to the flint when you knapp it, because all the results are actually taking place out of sight under the leading edge...just a few light tap-tap-taps and you'll be all set for another series of shots...only take 10-15 seconds.