Any kind of fat, beef, pork, chicken, lamb, goat, etc. (Suet works, too)
Dice all your fat up into small pieces and throw into the crock pot. You can fill the slow cooker to the brim, it will cook down.
Let the fat cook on low for 24 hours, stirring it throughout the day.
You cannot rush this process.
Strain the fat into glass jars.
Season the leftover fat bits heavily with salt and enjoy a unique tasty treat.