you actually put them in a trash bag, splash with ammonia, loosely close and leave outside overnight...the next day they are supposed to scrub clean easily. I bought the ammonia but haven't tried it yet.
I've tried this and it's not quite as "magic" as it's supposed to be if they are really grungy (mine were), but if they are lightly soiled, you should get the "magic." I did it several times and mine still had some grunge on them. That being said...it does work. It's inexpensive, so give it a try! Instead of trash bags, I used the 2 gallon zip storage bags.