Here is Flint Knapping 101. Take Rock A and smash it into Rock B (or anything else that is hard and unyielding). When the dust settles, pick up the sharp pieces and use until dull. Repeat as necessary.

That is basically how tools were made for thousands of years, and this while this technique is a far cry from the beautifully crafted Clovis points and Solutrean blades which are at the apex of this technology, it will work in a pinch. If you don't have flint or chert handy, don't despair. Glass works better than almost any natural product, even better than obsidian.