- Heat oven to 400°. Arrange bones on a baking sheet and bake for 1 hour or until the bones have browned.  
- Set the bones into the bottom of the crock-pot and add the juices from the baking sheet. Pour in the apple cider vinegar (used to help break down the collagen to produce more gelatin), fill the crock-pot with water and cook on low for 18-24 hours.  
- While cooking, the bones will produce a foam that can be scraped off and discarded. 
- When 2-4 hours remain, roughly chop the onion, garlic and celery. Add them to the slow cooker. 
- Once finished, strain the stock well to ensure no particles have been left inside the liquid. Refrigerate the stock and hopefully, once completely cooled, you will have a perfectly jellied bone stock! If there is a layer of fat on top, that can be scraped and discarded or saved to be used for cooking fat.  
- Pour into glass jars and use right away or freeze for a later time!