I sharpen my flints 2 ways.
First, take a large nail and a file. File the tip off the nail and then file a 1/16" deep step, 3/16" long on the end of the nail. Place the step on the cutting edge of the flint and knap with a downward movement. Tap the head of the nail with your patch knife or whatever...this will flake away the flint and produce a sharp edge. I know this is difficult to visualize, but if you care to give me a call, I can explain it better over the phone.
Second, I use my screwdriver and place the tip on the cutting edge of my flint. I apply enough to errode the cutting edge, making it uniform. This method will get you through a shoot, but is not my preferred method.
I purchase my flints from Cache LaPoudre Rifleworks in Ft Collins Colorado. Mike has the finest flints I've ever seen. My big gun usually gets about 100 shots per flint...more if I take it home and dress it up.
Hope this helps,