To trim a rack of spare ribs down St. Louis Style, start by lifting the flap of meat on the underside of the rack, known as the skirt, and remove this piece by cutting as close the bone as you can with a sharp knife. Next, slide the handle of a spoon under the membrane on the back of the rack, grab it with a paper towel, and peel it off.
The next step is to remove the lower portion of the rack, or the “rib tip” section. To do this, you can cut with the bone side up or the meat side up, your choice. I prefer to cut with the bone side up so I can clearly see the bones. Start on the end with the longest bones, at the end of the bone, and make a straight cut down the length of the rack. As you cut downwards the bones will be getting shorter and you will be cutting through cartilage.
Your rack of ribs should be nice and straight, with a uniform edge. You can smoke the skirt piece and rib tips as well, to be served as rib tips, or chopped and put into beans, stew, etc.