Cut suet or beef fat trimmings into 1-inch cubes and place into Dutch oven.
Cover Dutch oven and place on the middle rack of an oven preheated to 225°F.
After one hour, open the oven and carefully remove the lid from the Dutch oven using a potholder. Stir the contents of the Dutch oven, place the lid back on the Dutch oven, and close the oven door to allow the suet or beef fat trimmings to continue to render.
Periodically, check on the progress of the rendering process. After five to six hours, the fat should be rendered into tallow.
You will know that your suet has completely rendered into tallow when there is predominately a yellow liquid in the pan, and all that remains in terms of solids are brown bits ("cracklings") that have sunk to the bottom of the pan.
Remove the Dutch oven from the oven.
Strain the tallow from the contents of the Dutch oven using a fine-mesh strainer placed over a heatproof bowl. (See video.). The strainer will catch the cracklings, which you can reserve to eat as a snack.
Decant the tallow into airtight quart-sized jars. You can leave one jar at room temperature that you can use for cooking. For long-term storage, you can refrigerate or freeze the second jar. If you freeze the tallow, make sure that you leave at least one inch of headspace in the jar to allow for expansion.
At room temperature, the tallow will have a shelf life of at least one year.
If refrigerated or frozen, the shelf life of the tallow will be extended significantly.
For instructions on how to render suet or beef fat trimmings on the stovetop or in a slow cooker, see the "Recipe Notes" below.