Combine the chili powder, paprika, garlic powder, onion powder, and cayenne pepper. Rub all over the pork and store it in your refrigerator all night. Marinating the pork overnight allows the flavors of the ingredients to develop and infuse the pork with a massive amount of flavor.
When you're ready to cook the pork, remove it from the refrigerator and let it come to room temperature. This could take 2-3 hours, so take it out early!
Preheat oven to 300 degrees F. Place pork in a Dutch oven or roasting pan and roast until it falls apart easily. This should take about 6 hours. To test to see if the pork is done, take a fork and you should be able to easily shred off some pieces of pork.
When the pork is finished cooking, use 2 forks to shred the meat. Keep warm in the IP or Slow Cooker.