When a flint gets worn it's because the edge has chipped away and becomes irregular. A good flint will have an almost even edge along the face of the pan.
To sharpen you need to knap a new edge. Most of the time you can leave the flint in the hammer but some will remove to a hand or bench vise. Then using a light hammer, chip away under the face in little bites making small scallops under the face edge I support the edge with my finger. I sometimes use hand pressure and a nail with an 'L' shaped recess ground in to get more control. (grind/file away half the diameter of the nail partway down the shaft) use the nail to press off the material you want removed using thumb pressure.
All you want to do is even out the cutting edge while keeping the angle about the same.
It's easier to do than to explain... Look at a new flint and try to duplicate that edge.. Note it is a series of small chips except arkansas flints which are sawed. I wish I could show you. Maybe someone has it on YouTube???
Use a medium sized screwdriver and lightly tap the edge of the flint with the shaft of the screwdriver while the flint is held in the lock. They won't last for as many shots as a new flint will, tho. At least mine won't. Diamond honing stones will sharpen them too. The cut agate flints respond pretty well to a diamond hone. Don't waste your time with the cut arkansas stone flints, I've never gotten one to work well.