I use my 6qt Hamilton Beach programmable Crockpot for this one. Load up your crockpot with whatever carcasses you have available. I tend to separate them out so I can make Chicken Broth (backs, necks, feet, leftover wing bones etc.) or a Beef Broth (leftover steak parts, marrow bones etc.). Sometimes I will mix lamb and beef together. For the best gelatin broths get only the thick leg bones with marrow. I just use whatever I can get my hands on or whatever I have left over.
Fill the crockpot to cover all of your bone material. Add some salt across the top. I tend to go heavy with the salt. Also add about 1-2 Tablespoons of vinegar (this helps to break down the bone material).
Turn on the crockpot to high for about an hour and then drop it down to low for the rest of the time.
After a couple of days the liquid will get a nice darkened color and the bones will become brittle once they've expunged all of the good stuff. Siphon off the liquid into some jars for the freezer or keep some available in the fridge. I strain mine through a metal strainer into a pyrex measuring cup and then funnel into jars for freezing, or it can be canned with a pressure canner for longer term storage.