I mean it just takes time and a lot of poop. After a year or so bacteria and stuff break your poop down. So, what you are going to want to do is boil that huge pile of year old poop. Then strain it, but keep the water. Then boil the water down slowly. I’d probably let the last bit evaporate on its own. But, if you do it right. You wind up with some nice saltpeter crystals, or so I’ve heard. Also, I’ve heard tell that most poops will work but it really needs to be covered from rain.