Put 1kg of chicken bones in a large pot. Chop 1 small carrot, 1 leek, 1 onion and 1 celery stick together with a few peppercorns and one bay leaf, and add to the pot. Add about 3 litres of cold water and bring to a boil. Make sure you skim off and discard any scum from the surface. Reduce the heat and simmer for about 2 hours.
Strain and refrigerate overnight. Remove the layer of fat from the surface and freeze until needed.