Remove as much of the meat as you can from the turkey and then place the carcass in a slow cooker.
Add the celery, carrots, onion, garlic, and apple cider vinegar.
Fill the slow cooker with water.
Cook on high for 1 hour. Then reduce heat and cook on low for 12 – 15 hours: Strain the liquid. Save any bits of meat to use later in soup, but discard the bones and veggies.
Place the stock in a lidded bowl in the refrigerator.
After the stock has cooled, you can skim the fat off the top.
Divide the stock up into usable portions and refrigerate for up to a week, freeze, or can using a pressure canner.