Toss the pork with the cumin, salt, and pepper. At this point you can put the meat in a sealed container and refrigerate it for up to 2 days (it’s not necessary to season the meat ahead, but if you plan for it you will be rewarded with a deeper flavor).
Place the pork in a large stockpot or Dutch oven and add the citrus juices, garlic, and enough cold water to just cover the pork. Bring just to a boil, uncovered, over high heat, then reduce the heat so that the liquid is simmering. Simmer, uncovered, until the liquid has evaporated, 2 to 3 hours (a longish time frame, but it really varies, that’s a fact, and largely depends on the depth of your pot). You don’t have to monitor it much during this time; in fact the less messing around you do the better. You can skim off the slightly foamy stuff that will rise to the surface, but don’t worry about it too much.
Now the second part of the cooking process takes place, and here you need to pay attention. Cook the chunks of meat over medium-low heat in the fat that will have been released during the simmering. Flip the chunks every few minutes or so, so that all sides have a chance to be in contact with the hot bottom of the pan and brown up nicely. After about 30 minutes, the meat should be quite browned on the outside and start to fall apart—that’s how you know it’s done.
Remove the meat from the pan, let it cool a bit, and using your fingers or two forks, shred it into small pieces (see Note). Transfer the meat to a shallow bowl and serve with the warmed tortillas and the toppings you like.
Definitely do not over-shred the meat, as you don’t want only little stringy bits. The pleasure of carnitas is the nice chunks and the smaller pieces together. Then you get the best of the crispy caramelized exterior and the super-tender interior of the braised pork chunks.