I made a boat load of it for our housewarming party last week, a 6lb shoulder in fact.
One white or yellow onion
1 teaspoon ancho chili powder
1 teaspoon chipotle chili powder
1 teaspoon chili powder
1 teaspoon garlic powder
1 teaspoon achiote paste
3-4 tablespoons or bacon fat or lard (or shortening or veg oil if you desire)
salt to taste
Take the pork shoulder and place in crock pot, fill 1/3 the way up with water. Sprinkle pork shoulder with salt and top with a sliced onion.
Cook on low for 8-10 hours or high for 4-6 until the pork falls apart easily. Take out of crock pot and set aside to cook.
Strain the liquid left in the crock-pot with a sieve and return to crock pot, place on hot. Add in all the spices and let it come to a boil.
Once the pork is cook enough to handle remove any bones or large sections of fat. Discard.
Break the pork into large chunks and allow to dry as completely as possible.
Melt 3-4 tablespoons of chosen fat into a large skillet, and heat until shimmery but not smoking. Place large chunks of pork, and fry on all sides until it develops a golden brown crust.
Place pork back into crock-pot and turn into sauce. Turn crock-pot to warm until ready to serve.