In a small bowl, mix the salt with the rest of the spices. Cut the excess fat off the piece of meat and cover it with the spice mix, making sure to coat it all around. Cover the meat with kitchen plastic wrap and refrigerate for 2 hours.
Put a pan on the stove over medium high heat with some oil. Put the pice of meat on it to seal it all around. It will cook for about 2 minutes each side, until it turns golden brown.
From here on, the meat can be cooked in the oven or in a slow cooker. To make it in the slow cooker, put the meat in it, along with the onion, orange juice and tomato sauce. Cook over high level for about 3 to 4 hours or in the low setting for 7 to 8 hours until the meat comes apart easily with a fork.
To make the carnitas in the oven, preheat it to 302º F (150º C) and line a rectangular baking sheet with high walls parchment paper. Put the onion on the baking sheet and once the meat has sealed, put it on the same baking sheet along with the orange juice and tomato sauce.
Cover the baking sheet with aluminium foil and bake covered like that at 302º F (150º C) for 2 hours and a half. Then, take the aluminium foil off and bake for 1 and a half more hours (if the meat starts to overcook on top, turn it around).
Once the baking time has passed, pull apart the beef using a knife and a fork until you get bite size pieces or "strings".
Eat the carnitas in tacos, along with salsa or the way you prefer.