To prepare the bones, roast them in the oven at 400°F (200°C) for 30 to 40 minutes, or until they are browned, if you are using raw bones.
Get Cooking: Put the bones in a big slow cooker or stockpot. Add the apple cider vinegar, bay leaves, onions, carrots, and celery.
Add Water: Leaving approximately an inch of space at the top of the pot, cover the ingredients with enough water to completely submerge them.
Simmer: Once the broth reaches a rolling boil, lower the heat to a simmer and leave it uncovered for at least 8 to 12 hours, or up to 24 hours if you want the most flavor and nutrients.
Skim the Surface: As the food cooks, remove any froth or particles that come to the top.
To enhance the flavor, add salt, pepper, and fresh herbs during the last hour of simmering. Season to taste.
After cooking, drain the broth into a clean container using cheesecloth or a fine-mesh sieve. Throw away solids.
Chill and Store: Let the broth cool completely before freezing or putting it in the refrigerator to use later.