I wrote an article a while ago for Historical Trekking's "On the Trail" magazine on how I make flints. They might have some old issues.
The basic steps are
1) find some flint. Don't expect to be able to make gunflints from any old odd shaped piece of flint or chert. You need a chunk and to strike off blades that can then be made into gunflints.
2) Shape the piece until it is roughly tapered like a paper cup
3) Strike off blades from the "wide side" or "top" of the paper cup-shaped piece
4) Break the blades into gunflint-width pieces
5) Reshape those pieces as needed.
I now use a piece of steel rod, mild, about 5/8" thick and 6" long to break up nodules and strike off blades. Then I switch to a smaller rod about 3/8" diameter to break the blades and trim as needed. Different tools will work fine. The harder your "baton" the more curved your blades will be where they detach from the core. This is called a "cone of percussion". I need to use steel because the rock I use is tough unless heat treated.
Expect to start with a peck of rocks and get 3 or 4 usable gunflints the first try. With experience and a good nodule to work, say the size of a painted turtle shell, you may eventually get 20-30 gunflints from one rock.