Combine the bones and water in a large stock pot. Bring the mixture to the boil. Maintain a low boil for 24 hours, adding water to keep the bones submerged. Remove and discard the bones. Allow mixture to simmer another 21 to 22 hours. Strain the liquid through a fine-mesh sieve and return to the stock pot. Bring to the boil and cook until liquid has reduced to about 2 litres.
Transfer the stock to heat-proof containers; freeze. Once frozen, there should be three easily distinguishable layers. Remove and discard the top-most of those three layers. Thaw remaining portion to use.
Beef soup or stock bones are available at your local butchers. You may need to special order them.