Here's what you need:
shaving cream (plain white kind)
throw away pan or something to put the shaving cream in
toothpicks and Popsicle sticks
white lunch bags
Start by spraying the shaving cream into your tray, then smooth it pretty flat.
Next, add a few drops of the paint colors you want to use. I would suggest only 2 or 3 colors.
Using the toothpicks and Popsicle sticks, swirl the paint around in the shaving cream to make a sorta marble look. Really, the less swirling you do the better the look.
Next, place a bag down in the tray and gently press.
Then carefully pull the bag up. There may be some globs of shaving cream on the bag, I sorta scraped them off some using a Popsicle stick. But, don't worry too much about it. After the bags dry the shaving cream just falls right off.
Sit them out to dry overnight.
Once dry, rub the bags gently and the excess shaving cream will flake off. It's best to do this over a trashcan or something that will catch it.
You will end up with this cool 'marbly' design!