Yes, a strip of suede leather is needed, the width of the flint. A bare flint will not stay put in the jaws. Orientation of the flint, be it bevel up or down, depends on the lock. You just get to figure it out for best performance from the particular lock. If the flint happens to be too long, disallowing the frizzen to close fully while at half cock, cut out a notch in the middle center of the leather strip to allow the flint to butt fully back against the jaw screw. A small gap between edge of flint and frizzen face at half cock is what you're looking for.
You can find patterns to cut printer paper into suitable sizes to form around a ball diameter wooden mandrel to make your cartridge, or the Jefferson Arsenal offering is certainly a good choice. You're gonna have some fun and give it time to figure out what the gun likes. That's what is so much fun about these guns, they require establishing a "relationship" between gun and shooter.
