I like to pick a package that has some good fat marbling. Fat marbling helps to keep the meat moist plus adds flavor to the meat. If you are turned off by fat remember if the ribs are prepared properly the fat will render down and melt into the meat. I take the ribs out of the package and inspect the ribs on all sides I will trim off any loose fat pieces. I flip the ribs over and remove the small flap of meat that will be sticking off the back. I typically save this and add it to the smoker with ribs and use it as an appetizer for me while I am smoking the ribs.
The next step is to find the end of the longest rib bone. I insert my large kitchen knife and cut straight across the rack. You have just turned your spare ribs in to St. Louis cut.