Note: You can buy good quality lard from a local butcher or Mexican market. I don’t recommend the hydrogenated lard that’s sold in bricks.
Cook the pork in a slow cooker. Sprinkle the pork generously on all sides with salt (you’ll need about 2 teaspoons). Scoop the lard into a 6-quart slow cooker and turn on high. When the lard is melted, fit in the pork in a single layer (there should be few gaps between the pieces and they should be barely covered with the fat). If you’re using the flavorings, sprinkle with oregano and black pepper, then fit the garlic, bay leaves and orange slices between the pork pieces. Cover and cook until thoroughly tender, 3 to 4 hours. Turn off, uncover and let cook to room temperature, about 2 hours.
Or cook the pork sous vide. Place the pork in a large bowl and sprinkle the pieces generously on all sides with salt (you’ll need at least 2 teaspoons). Spoon on the lard, then, if using the flavorings, sprinkle on the garlic, bay, oregano, pepper, and orange slices. Mix thoroughly. Transfer the mixture to a heavy 1-gallon zippered or vacuum-sealable plastic bag(s). Set up your pot for the sous vide water bath and adjust the circulator temperature to 185 degrees. When the water reaches temperature, either vacuum seal the bag(s) and slide into the water or lower the unsealed zippered bag of pork carefully into the water until nearly all air is forced out, then zipper-seal the bag. (If you’re careful to remove almost all air from the bag before sealing it, the bag should float freely in the water, rather than bob on top). Cover the pot and the immersion circulator with plastic wrap to prevent the water from evaporating. Cook for 8 hours. For the best texture carnitas, plunge the bag of pork into an ice water bath when the cooking time is up.
Browning and serving. Remove the pork from the fat in the slow cooker or from the bags; you’ll notice that there will be pork “juice” (what commercially is called “purge”) below the fat. Lay the pork on a rimmed baking sheet lined with paper towels and pat it dry. Scoop a few tablespoons of the fat (but not the juice) into a very large (12-inch) skillet (preferably cast iron) and set over medium heat. When the fat is hot enough to shimmer, brown half the meat in an uncrowded layer, turning it until richly golden. Remove to a rack set over the rimmed baking sheet and keep warm in a low oven. Brown the rest of the meat. Arrange the carnitas on a serving platter and sprinkle with salt (this is a good place for a finishing salt like Maldon). Serve with the guacamole, salsa, limes and warm tortillas. Encourage your guests to pull apart the golden pieces of deliciousness, fitting several pieces on each tortilla, before topping with guacamole, salsa and lime.
Working ahead: The slow-cooked or sous vide carnitas can be wrapped and refrigerated for several days before browning.