I make Prime Rib for Thanksgiving and this is what I do. Take the prime rib (boneless) out of the refrigerator about 1 hour before you are going to roast it. Rub in kosher salt and fresh ground pepper, you can add garlic powder and rosemary if you desire. Roast for 10 minutes per pound at 325 degree Fahrenheit until 125 degrees on a thermometer. Take out of oven and let it stand for 15-20 minutes before carving. The cooking time is an estimate because it can vary with the thickness of the meat. The biggest mistake people make is overcooking; if someone desires it cooked more, put the slice in a frying pan and cook that slice on top of the burner.
I serve it with a horseradish cream. Whip unsweetened whip cream and when soft peaks form and add grated horseradish.