Cover the carcass, stripped of all the meat, and any other bones with cold water, along with the vegetables and herbs. Don't worry if you don't have all of these ingredients.
Bring to the boil and then simmer gently for a couple of hours. Don't boil it or you will end up with a tasteless stock, it requires a gentle simmer.
Remove the big bones and then sieve. You will make 1-2 litres; ideally it should have a slight jelly texture when cold.