I filled my mom's slow cooker and mine with bones. Plus water, carrot tops, onions, peppercorns, salt, and - most importantly - a splash of vinegar. The vinegar draws the calcium out of the bones into the stock. Amen for calcium! It's not just in milk!
I simmered the whole business for 4 days and nights. Truthfully, it doesn't smell all that great.
Then I let it cool down, strained off the fat (tallow), and canned the stock with Rebecca's pressure canner because my freezer was chock full of beef. Oh my. I had never had a bomb (pressure canner) in my kitchen before, but Rebecca and the owner's manual were both clear and reassuring.
I think I'm going to try to make soap with my little bit of tallow - why not? It's a lovely pale green from the carrot tops.