Cut the fat trimmings into small cubes.
Place in slow cooker and cook on low for 6-10 hours (depending on the size of your cubes, size of your slow cooker and amount of fat you have).
Remove the cover and stir periodically to check on how it’s rendering. There will be some pieces of cooked beef in it.
Line a fine mesh strainer with cheesecloth and set over a bowl or a large glass measuring cup. Strain the cooked pieces of meat from the rendered fat into the bowl.
Divide among glass mason jars. Store in fridge or freezer.