Place the bones in your slow cooker, filling about 3/4 of the slow cooker.
Chop vegetables and garlic (no need to peel). Add additional rinsed vegetable scraps and egg shells if desired. You'll be straining these out before storing the broth.
Fill the slow cooker with water and season with about 1 tsp of salt (or to taste).
Add apple cider vinegar (no worries -- you won't notice the taste in the finished product).
Cook on low for for 18-24 hours.
Strain everything through a fine mesh strainer and discard bones and vegetables.
Cool the stock and store in an airtight container in the refrigerator for up to 4 days or in the freezer for 6 months.