Turn off all fans and air conditioning within your home, as these can misguide you on your search for the foul scent. The air should be as still as possible as you begin locating the source of the odor.
Detect where the odor is the strongest. Make several rounds around your house with your nose held high and note the area where the scent is the most repugnant. This will bring you straight to carcass of the rat.
Don a pair of safety goggles, a mouth mask and thick gloves prior to locating the rat. Set up a ladder beneath your attic's most convenient access point. Climb the ladder, lift the entry door and climb carefully onto the wooden rafters of your attic.
Search between the rafters with a flashlight until you locate the carcass. If you are having trouble locating it, use your sense of smell to guide you -- gravitate toward areas where the stench is strongest. Be sure to walk only on the wooden rafters as you search.
Pick up the rat and place it in a zip-close plastic bag. Place the bag into a second and seal it shut. Hand the bag down to an assistant.
Spray the carcass' resting place with an enzyme-based biological cleaner to sanitize and deodorize the area. Cleaners of this sort are available at most industrial cleaning supply stores.
Exit the attic and descend the ladder carefully. Place the bagged carcass in an outside trash receptacle or phone your local animal control agency for pickup.