Simply place knuckle bones in a large stock pot. Fill with water and set to low for 12 hours ish. Since the pot will be over heat for at least 12 hours, you want to be sure there's enough water to compensate for some evaporation.
I usually have it on low all day and before going to bed I turn the heat off so that when I wake in the morning, the fat, scum and blood solidify at the top and is easy to scrap off. I use a cheap scrapper I got from the Asian market, it's great for this :)
After scrapping the fat off, I set to low for one more day. You don't have to do it that long and 12 hours or until the broth has a nice milky white color is good enough.
And you're done! Yummy bone broth! :)