Take the ceramic bowl out of your slow cooker. Paint the bones in olive oil and place them in the bowl until it is nearly full.
Roast bones at 350 degrees in the oven, any higher and your risk cracking the bowl. Times vary depending on the animal, less for chicken, more for lamb, more for pork, and even more for beef. You want them to brown a bit.
Take the bowl out of the oven and let it cool, putting the water in too soon can also cause the bowl to crack. Once cooled chop an onion in half and place it in the bowl. Take a whole garlic clove, cut it in half, place it in the bowl.
Fill the bowl with water and put it in your slow cooker on high. Times vary depending on the bones and how long it takes for your slow cooker to get to a boil. 2-3 hours of boiling for chicken, 6 for pork, 9 for lamb, 12-24 hours for beef especially if you're using the neck bones.