Put the leftover chicken bones from a whole chicken in the slow cooker along with the juices, fat, etc.
Cover with water up to about an inch below the top of the slow cooker. Add a few tablespoons of apple cider vinegar, about 10 peppercorns and some celery from the freezer (and parsley, if I have it).
Turn the slow cooker on high and let it cook for several hours. Before I go to bed, I turn the temperature to low and let it cook all night. You can cook it anywhere from 12 to 24 hours.
Turn the slow cooker off and let it cool for a bit. Then strain the stock.