Place the beef fat trimmings into the slow cooker.
Set the slow cooker to low heat.
Cook the fat for 8 hours, stirring occasionally to prevent sticking and promote even rendering.
After 8 hours, the fat will have melted into liquid and the remaining solids will be crispy cracklings.
Line a fine mesh strainer with cheesecloth and place it over a heatproof container.
Carefully pour the rendered fat through the strainer to remove any solids.
Allow the strained tallow to cool to room temperature, then cover and refrigerate until solidified.
Store the beef tallow in an airtight container in the refrigerator for up to 3 months or freeze for longer storage.