The bean hole should be 2 1/2 to 3 feet deep, depending on your pot. The hole should be big enough around to have a 6 inch space between the pot and the edge of the hole on all sides. To help hold heat, put some old tire chains or stones in the hole before starting the fire.
Start the fire and keep it filled with good dry hardwood. Let it burn for about 3 hours. The hole should be at least 3/4 full of hot coals. After the fire has been going for about an hour, place the beans in a large pot, on the stove with water to cover. Bring to a boil and cook until skins roll back when you blow on them, about 45 minutes. Watch closely, because they will get mushy if left too long.
When the hole is ready, cut the salt pork in to 2 inch wide and 1/4 inch thick slices. Place them into the bottom of the bean pot. Peel and cut the onions in half; lay them on top of the pork. Pour the beans and their liquid into the pot, then mix in the molasses, black pepper and dry mustard. Slice butter and place on top. Add enough boiling water to cover the beans by one inch. Cover the top of the pot tightly with aluminum foil so that it goes down over the sides by at least 2 inches. Place lid onto bean pot.
Before putting the pot into the hole, remove about 1/3 of the coals using a shovel. Remove and discard any burning pieces of wood. Place the bean pot into the hole, and put the coals from the hole back in around the sides and over the top of the bean pot. Now start filling the hole in with the dirt, packing it down with your feet as you go. You should end up with about 2 feet of dirt covering the pot. Cover the place where the beans are buried with a tarp or piece of metal to keep out rain.
Let the beans stew overnight in their bean hole. Carefully dig them out the next day and enjoy!