First you need to clean them from the stuffing to avoid making the liquid cloudy. Then for 1 BBQ chicken, you put the bones with 3 litre of cold water in a large pan and cook over medium heat.
Bring them to simmer, then turn the heat down to low and let it cook for 1.5 hours to reduce the liquid.
Strain the liquid through a very fine sieve.
You can use this chicken stock for cooking straight away, or you can freeze them for later.
For extra taste, you can also add carrot, celery, leek and onion. Just boil them together. But if you do want to add anything at this moment, you can always put the vegetable later when you cook like I always do. This way you make sure you put what you need according to what you want to cook at that time.