Cut the pork into 2-inch (5cm) cubes.
Heat the lard or oil in a large, heavy skillet or dutch oven over medium-high heat.
Fry the meat to lightly brown all sides (about 10 minutes). Fry it in 2 batches if necessary.
Add the onion and return all the meat to the pan if you browned it in 2 batches. Fry for another 10 minutes, until the pork is well browned, stirring occasionally.
Add the marjoram, thyme, bay leaves, orange chunks, milk, salt, and pepper.
Cover the pot, reduce the heat, and simmer for 30 minutes or until the pork is tender. If cooking over a campfire, move the pot to a cooler part of the fire so that it just keeps cooking at a nice gentle bubble. There will be a lot of liquid in the pot.
Remove the lid and cook until most of the liquid has evaporated (about another 30 minutes), stirring occasionally.
Remove the herb stems, bay leaves, and oranges, and scoop out or drain off some of the excess fat accumulated at the top of the liquid.
Increase the heat (or move the pot to a hotter part of the fire) and fry the meat until the edges get dark and crispy. If there is too much liquid in the pot for it all to evaporate and let the meat get crispy, remove the meat chunks to another skillet or drain off the liquid, and then cook them until crispy-edged.
Use two forks to pull the pork chunks apart into thick rope-like shreds. Season with more salt and pepper to taste, if desired. Keep the carnitas warm and covered so the meat doesn't dry out. Serve in tortilla shells with your favourite toppings.