Preheat the oven to 250 degrees. Season the roast and cook fat side up uncovered in the oven. If you use an electric roaster you will bake it with the cover. If you bake a prime rib in the roaster pan, you will not cover the meat with the lid. A meat thermometer is recommended to ensure the desired doneness. Remove the roast from the oven when the temperature hits 138 to be rare, 148 to be medium, 158 to be well done. Larger roasts may cook faster than times listed, so watch over your thermometer. Your roast will continue to cook approximately 5 additional degrees once removed from the oven or roaster. Let the roast sit for at least 15 minutes before you start to carve it. Oven temperatures vary so much, please keep your eye on the temperature of the meat.