First, there are two muscles that make up the tenderloin. Locate the vein of fat that separates the two and gently pull them apart at the narrow end of the loin until they begin to separate.
Using your knife, carefully cut through the membrane at the wide end of the loin so that you can finish separating the two muscles. Save the smaller muscle for use in stew, chili, or other recipes.
Turn the loin over to the rib side.
Cut off each of the fatty lumps there, making sure to cut each one off level with the last, leaving a smooth, uniform surface free from bumps and gouges.
Save these meaty lumps for use in other recipes.
Trim about 3-4" from the narrow end.
There is still a lot of fat and membrane that needs to be cleaned from the loin. Slide the tip of your knife just under the surface of the membrane, making the cut as shallow as possible.
With the blade of your knife angled slightly upwards, slide it along the underside of the membrane until it separates from the loin.
Holding the loose end of the membrane, slide your knife the other direction in the same manner.
Continue removing strips of membrane and fat until the loin is clean.
At the wide end of the loin there are three muscles that come together, and membranes that connect the three. Pull the smaller muscle towards you, exposing the membrane.
Using the tip of your knife, cut the membrane out in the same manner that you removed the others.
The cleaned loin should look like this.
Starting at the narrow end, cut the loin into desired portion sizes.
Weigh each portion for accuracy.