Flintknapping is a process of reduction. You have a large nodule. You fracture off a spall or flake. The thick parts of that flake are thinned out with percussion, then pressure flaked into a point.
I am going to try to explain a few basic concepts behind striking 'predictable' flakes off of a core with a hammerstone. Your intial core needs a flat striking surface...this is called the 'platform.' Some oval nodules would need to be broken in half, or one end knocked off, if necessary to produce a flat platform.
The angle between the striking platform and the adjacent face should be 90-degrees, or less, to successfully strike flakes off. An obtuse angle of more than 90-degrees will not work. The obsidian core pictured has a platform, and adjacent side, at slightly less than 90-degrees as an example.
The next concept is important.
If you have ever seen a BB hit a glass pane...it produces a cone of about 100-degrees. This is called the 'Hertzian cone.' It is consistant. The picture shows an obsidian core which has been struck, with insufficient force to crack the core, that left evidence of the force generated into the rock- forming the hertzian cone. Cool, huh? Early man was intelligent and took note of cause and effect to be able to develop stone working skills to high levels.
Now, in order to strike thin predictable flakes off the core, and not just make gravel, I mentally apply this concept of the hertzian cone. I judge what angle I need to strike a glancing blow, that one edge of the cone of force will intersect into the edge of the rock face. I prepare my platform, grind it some with the hammerstone, to make it more abrasive, so that it make grip a little when I make contact.
There is one more principle I can apply - energy follows ridges. You can see some of the ridges formed by the removed flakes on the next pic. My flakes will be longer if there is a ridge to follow. If there is no ridge, the flake may end up somewhat clam shell shaped. Depends on what you want. Aborigninal man at times took great care in the preparation of the core to remove specialized flakes, as in the European Levallois and Mousterian technics.
Here is a top shot of the removed flake. You can see ridges left behind, which set up for more long flake removal.
Here are several flakes, about 3 inches long, 2 inches wide. The one on the right struck a pebble and lost a section when it detached. This is how butchering flakes were made - basically one blow stone tools. The edges are very sharp. The middle and right flakes have a curled base edge. This could be retouched a little, and sides dulled, to make hide scrapers. The one on the right could have the top retouched to make a perforator/awl for making holes in a hide to run lacing.
By judiciously striking somewhat thin flakes off of the core - I have less work to thin the flake for a point, and I can remove more usable flakes and blades per pound of material (stone age economics).