Gunpowder is a mix of 75% potassium nitrate, 15% charcoal and 10% sulfur.
You need to grind each of the components into a fine powder.
IMPORTANT: Grind each one separately! If you grind them together you might end up testing out your gunpowder on yourself.
Once ground up, they need to be very finely mixed together. This is an important part. Ideally you need to put the mix into a tumbler to get the parts well mixed. If you don’t have a tumbler, then just take some time and mix very well (remember – don’t grind at this point).
Once its mixed this is known as ‘meal powder’. The ideal form of gunpowder is in the form of grains; the size of the grain determining the speed at which it burns, the smaller the better. To get small grains you can mix the power with water to form a paste and then press the paste through a fine-grained sieve. You should then (ideally) tumble the grains to make them smooth.
IMPORTANT: If you tumble the meal powder, DO NOT tumble with certain metals like steel and iron (in fact anything that can create a spark, which include many plastics), wood would be OK to use.