Place the bones and feet if you use them, into the slow cooker, add the vinegar and water to cover and leave to stand for 30/60 minutes.
Add celery and onions, cover and cook on low for a day, you can add more water if needed.
Remove the bones and feet with tongs and if this was just bones, strain through a sieve and use as a base for soups, curries, bolognaise sauce, gravies, casseroles or even as a tasty savoury drink.
If you cooked with a whole chicken. Remove the feet and any hard onion skins and leaving the meat into the broth, boil any other vegetables you would like to add and eat as a casserole/stew.