Place all the ingredients in a large pot and cover generously with water.
Bring to a boil, not a simmer, and cook for at least 6 hours.
Add some water if the bones are uncovered.
Skim regularly, every 30 minutes at the start and after longer as time goes on.
Ideally, cook for 24 hours, but nobody got time for that. You are aiming for about 2 liters at the end.
DONE!