Potassium nitrate or saltpetre is one of the main ingredients in gunpowder, popular since the Middle Ages as a weapon of war until smokeless powder supplanted it in the late 19th and early 20th centuries. Potassium nitrate still finds use today in fertilisers. It's typically produced by combining potassium chloride and sodium nitrate; for most of its history, however, it was produced using decaying organic matter like dung or urine. You can still make potassium nitrate using the older process, but be aware before you start--it's a time consuming job. The end product can also pose safety hazards under certain circumstances.
Form a large heap of manure atop a waterproof base (a piece of plywood or a layer of clay will work). The size of the heap will depend on the amount of potassium nitrate you want to produce; 4 to 5 feet high is generally best.
Mix in the green plant matter and ashes. Your heap should be roughly one-half dung or manure and one-half organic material and ash.
Build a roof over your dung heap by constructing a cinder block wall around the pile, then suspend the tarp over it to protect it from the weather.
Water your heap each week with plenty of stale urine. A crust of whitish-yellowish crystals should eventually begin to form on the surface--typically after about four to five months, depending on the climate. As soon as you see this crust, cease adding urine to the pile.
Collect the crystals by scraping them from the top of the pile; new crystals should then form in their place, typically fast enough that you can harvest more of the crystals within a few days or another week.
Once all the crystals have been harvested and new layers of crystals have ceased to form, take one of the two-gallon buckets and drill several holes through the base. Place a piece of cotton cloth over the holes then add a quarter-inch layer of ash. Suspend the bucket over a pan. Now fill the bucket roughly three-quarters full with some of the remaining dung mixture from your heap and add boiling water. The boiling water will slowly filter through into the pan, dissolving potassium nitrate and various other compounds as it does.
Pour the water from the pan through a paper filter like a coffee filter to separate the potassium nitrate from the other salts. You can use the same procedure for the crystals you harvested earlier. At this point, you should have some old-fashioned potassium nitrate.
It's generally much easier to buy potassium nitrate than to make it yourself; it's often available as a fertiliser online or at some home and garden stores. The pamphlet "Instructions for the Manufacture of Salpetre", published in 1862 and available online (see Resources), gives a good overview of the techniques Europeans and Americans used to manufacture potassium nitrate before the advent of more advanced techniques in the early 20th century.
Potassium nitrate is a strong oxidising agent; in and of itself it is noncombustible, but it can greatly accelerate combustion of flammable materials. Keep it well away from heat and fire and do not store it together with other chemicals. It can produce toxic oxides of nitrogen in fires and when inhaled acts as a mild eye and throat irritant.