Melt the lard in the body of a pressure cooker, a large, Dutch oven or any other suitable cooking vessel. Bring the grease up to 275 degrees F. Using tongs, submerge the meat into the grease. Monitor the temperature, as it will drop considerably following the addition of the pork. Monitor it until it stabilizes. Adjust the flame to keep the temperature within a range of 260-275 F.
Periodically rearrange the pork with tongs. Continue to monitor the temperature.
Fry the butt for about 1 1/4 hours or until tender. (At this point, if the meat is not crispy-brown, remove it and increase the heat to 325 F. Return the pork to the grease and fry for about 1 minute - until golden-brown.) Drain the pork on paper towels. Salt it down and season with a self-determined amount of Guajillo chile powder.
Pull or break the meat apart into strands. Stuff pre-warmed tortillas with the meat. Top with chopped cilantro and onions. Serve with green chile salsa.