Place raw bones directly into the slow cooker. Use as many bones as will fit comfortably without overcrowding. Add chopped celery, peeled carrots, parsley, and fresh thyme. Surround the bones with the vegetables.
Pour in the raw apple cider vinegar. This helps extract beneficial nutrients from the bones. Add cold water to the slow cooker, ensuring the bones are fully submerged but leaving about an inch of space at the top to prevent overflow.
Cover the slow cooker. Set to Low and cook for 24 hours (ideal for nutrient extraction), or set to Medium and cook for 12 hours. Stir occasionally if you’re around. It’s normal to see bubbles and fat solids form on top this means it’s working!
Turn off the slow cooker and let the broth cool slightly. Remove and discard all cooked bones (never feed cooked bones to dogs they are dangerous).
Skim off the hardened fat layer after chilling, if desired. Strain the broth through a fine mesh sieve to remove bone fragments and vegetables.