Place beef bones in a 10-quart capacity slow cooker.

Add water to the slow cooker, ensuring the bones are submerged by about an inch.

Turn on the slow cooker and cook for at least 12 hours on the low setting. For 24 hours, you may need to re-run the cycle twice or three times, depending on the slow cooker.

Skim the fat that rises to the surface occasionally.

Remove from heat and allow to cool slightly.

Discard solids and strain remainder in a bowl through a colander.

Refrigerate for a few hours before removing the top layer of fat with a spoon or fork.