Clean the blood and impurities: If the bones have blood remaining on them, soak them in salt water for 2 hours. Use enough water to just cover the bones. For one kilogram of bones, about 1-2 tablespoons of salt would suffice. When the blood is cleaned and the water turns red, throw it away.
Put the bones into a pot full of water: Wash the bones and put them in a large pot. Great if they are cut to the right size. If you got them from your local butcher, ask him to cut each one into several pieces so that it cooks more easily and the marrow releases nicely.Pour water over the bones, about twice the volume of the bones. For the first type of cooking, however, you can use less water. Toss in onions and bay leaves or cinnamon sticks: Take two onions and chop each one into 8 pieces, then add them to the pot in addition to 2 bay leaves or cinnamon sticks. If you wish, you can also add a few cloves of garlic.
Increase the heat for about 10-20 minutes to get the water to a boil: By increasing the flame, dark-colored foam (bubbles) rises to the top. Spoon it off with a scoop and throw it away.
Reduce the heat: Lower the flame so that the beef bones cook for about 12 hours and the fat leeches out. The heat should be gentle. You can put the lid on the pot.
Let the bone broth cool down, then lift off the fat into another container: Turn off the flame and let it cool down for 20 to 30 minutes. Then use a scoop to lift off the oil on top. You can tilt the pot a little to get a better result. This considerable amount of oil obtained would have many uses in cooking.
Separate the bones and throw them away.