In the largest pot you own (or in your slow-cooker), add the bones, bay leaves, and vinegar. Fill with water. If making this for people and not for pets, add the onion and peppercorns.
If making this for people, add salt to taste, being careful not to burn your tongue!
Allow to cool, and then skim the fat off the top and discard. Strain and pour into jars to store. If storing in the fridge, use within a few days, or store in the freezer for several months.
For determining quantity, you'll need to let experience and personal preference be your guide. As a general starting point, figure about two pounds of bones per gallon of water.
Two tips for storing in the freezer: First, you can freeze some in ice-cube trays and then you'll have some small amounts ready-to-go when you need them. Second, if it's at room temperature, you can pour into a zip-top bag and freeze flat in the bag -- just be sure the bag is sealed very well!