In a large stock pot or Dutch oven, place the bones, any onion scraps, apple cider vinegar, bay leaves, peppercorns, salt, garlic (if desired), and other herbs (if desired).
Fill pot with water until it barely covers the bones.
Bring to a simmer over medium-high heat, then reduce heat to a simmer. You want it to just be barely bubbling. Cover with the lid slightly ajar. After the first 30 mins to 1 hour skim off any impurities (if needed). Add other vegetable scraps in the last 2 hours of cooking time.
Cook for 24 hours. If leaving the stove on overnight makes you nervous, you can place the whole pot (covered) in the fridge instead, and continue the cooking time in the morning.
Once cool enough to handle, strain broth through a fine mesh sieve, and transfer to jars for storing in the fridge or freezer. Leave a thin layer of fat on top but skim the rest (if needed).
Once chilled, the broth should be jiggly like jello. Scrape off the layer of fat when ready to use.