what you wanna do sounds kinda mean, like drowning but without the water. just place the rat on its belly on the counter, put the handle of a wooden spoon is what i use across the back of its neck and give its tail a tug upward to break the neck. its instant. or you can go drop a dollar and 35 cents on a pound of dry ice if you dont want to get your hands dirty.
if you dont know how to do that, just put dry ice in a bowl, place it in a smaller tub with the rat and pour water on the ice and close the tub. check back in a couple min and it will have passed.
i do it with dry ice just about every week since i get my feeders live from a local breeder. best way and probably the cheapest way to euthenize them in my opinion.