Place the beef suet or fat trimmings into the slow cooker.
Add 1 cup of water to the slow cooker to prevent the fat from burning during the initial rendering phase.
Cover the slow cooker with the lid and set it to low heat.
Cook the fat for 8 hours, stirring occasionally every 2 hours to ensure even rendering.
After 8 hours, check that the fat has fully melted and the solids have turned crispy and brown.
Carefully strain the rendered fat through a fine mesh strainer or cheesecloth into a heatproof container to remove any solids.
Allow the strained tallow to cool at room temperature until it solidifies and turns creamy white.
Store the beef tallow in an airtight container in the refrigerator for up to 3 months or freeze for longer storage.