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!