Preheat oven to 300 degrees F.
Cut pork into 3" chunks, discarding any big pieces of connective tissue, but keeping all of the fat.
In a large, wide oven-proof pot, dutch oven, or cast iron skillet, put the pork chunks into 1 layer.
Sprinkle with salt, pepper, and cumin powder.
Scatter the garlic and onions on top.
On the side, combine orange juice and chicken broth (or water).
Pour over the pork until 3/4 submerged.
You want to be able to see pork on the top so you may not use all of the liquid.
Pour over the vegetable oil.
Place pot into oven on middle rack or so there is even space all around it.
Let cook for 4-5 hours, checking every 30 minutes or so. It should take about 1.5 hours for the liquid to start simmering and the fat to start rendering out of the pork.
After 1.5 hours, if the liquid is not simmering, turn up the oven by 50 degrees to kick start the process then turn it back down after it is simmering.
Turn the pieces over every 30 minutes. Eventually, you want the pork to be frying in its own fat with some of the liquid reduced. If it looks too dry, add some more chicken broth/water or OJ. Keep checking it with 2 forks to see how easily it pulls apart. When it falls apart, it is done. But make sure the fat has rendered out! The one thing you don't want is a bit chunk of fat on top of the pieces of pork.
Remove from oven, allow to cool slightly then shred with 2 forks or tongs, adding a bit of orange juice if it looks too dry.
Serve with corn tortillas, pickled red onions, slices of avocado, or your favorite taco toppings.