Glass works much like obsidian, which is volcanic glass. With thinner pieces of glass, you would mostly pressure flake it. With thicker or irregular pieces, you would need to do some percussion. Flint is a bit harder/tougher, depending on the variety, but all knappable material breaks the same way, with a conchoidial fracture. The only difference is the amount of force needed to take a flake off in some materials is greater than in others (if you discount things like inclusions, varying textures within the same rock, etc.)