Couldn't be simpler -- low & slow always wins. You need a prime rib roast, a meat thermometer (preferably a remote monitoring one so you don't have to keep opening the oven & poking) and an oven.
Put the oven to 200F or less. You can even do it at 150. The less temperature, the more time and the more tender. 200 is a good balance.
Bring internal temperature to 125, remove from oven, and leave the probe in. Let rest as temperature continues to climb. When it tops out and begins to decrease, let rest another 10 minutes or so, cut & serve with a high-quality beef stock slightly seasoned & reduced, and horseradish.
Here was my Christmas dinner roast, photo courtesy of my dad with his new digital SLR (click on the image for the larger, high-resolution version). As you'll note, there's no gray band around the outside, the telltale sign that a roast has been cooked too rapidly.