Here are the directions that I have saved. I haven't tried it yet though.
Smackin' Acetate - This is a super easy and fun technique. Take a piece of acetate (or clear transparency film, made for overheads) scribble on it with your water based markers. (I use the Many Marvelous Markers from Stampin' Up!). When scribbling on the acetate make sure not to blend to many colors together as it will give the background a muddy look. One you have all the colors added spray the acetate with water. I use a fine mist spray bottle. Than place your cardstock on the acetate and lift it off. Dry your cardstock thoroughly and than finish the card.
These directions came from this website:
http://www.stampincrazy.com/techniques.html
HTH-
Amber