Repost - How to make Prime Rib for Beginners. Use this formula for cook time. For every pound multiply by 5 and cook at 500f degrees then turn off heat and leave in oven for 2 hours. Step one - salt the prime rib and place uncovered in the refrigerator overnight. Step two - take out of the refrigerator for 2 hours and pat dry with a paper towel. Step three - trim some of the fat on top. Step four - tie tightly with kitchen twine. Step five - season generously and place on cooking rack. Step six - preheat oven to 500f degrees and roast 5 minutes per each pound for medium rare. Internal temperature should be around 125f. Then turn off heat and leave in the oven for another two hours. Do not open the oven during this whole process. #primerib