Black powder or gun powder as it is know today was produced by grinding charcoal, sulfur, and saltpeter. With a ratio of 70% saltpeter, 14% charcoal, 16% sulfur. This process was long a grueling one, especially the gathering of resources.
The saltpeter was the hardest to get because it was produced by soaking manure in urine before adding a source of potassium and water to get the white powdery substance. Charcoal was the easiest of the three substances to get because nearly every house was burning wood and would most likely have charcoal left in the fireplace. The sulfur could be found in natural deposits.
Once the materials were gathered the powder smith would use a mortar and pestle to grinder the materials together to a fine powder. The fineness of the powder will change how powerful it is. The finer it is the stronger it will be because the materials are mixed together more.