Just did one for Christmas. A piece of really good beef that large is really hard to screw up, so relax about it. It's my favorite thing to cook due to how easy it is and the presentation is much more impressive than the amount of work in it. Mine was a 7-rib roast of about 16 pounds, but weight really doesn't matter as much as temp with a prime rib roast, it's all about temperature once the brown is on. I rub mine with olive oil then Kosher salt and fresh cracked pepper. Be sure to hit the ends. Leave out for at least a couple hours before cooking. Pre-heat oven to 425 and wait 30 minutes for it to truly heat up. Insert either a remote meat thermometer or one you can read through the oven door into the middle section of the roast. Put in roast for 20 minutes. Do NOT open the door, reduce heat to 325. Cook until internal temp is 120-125. Ends will be medium, then a small portion of medium rare, then a huge chunk of delicious rare. (It took about 3 hours with my roast) Let it rest on the counter loosely draped in foil for about 30 minutes before carving, the internal temp will rise to about 135 or so while it rests. Some folks like gravy of some sort, like for on the potatoes or whatever. I like adding the roasts drippings (with most of the fat skimmed off) to a can of mushroom gravy and heat up on the stove top. If not already done by the butcher separate the ribs fro the roast then tie them back on with butcher's twine before cooking. The bones add a lot of flavor but having them easy to remove after cooking makes carving easier. I hope this helps!