I've done the "crinkle and pounce" method described by pp, but I've also done this...
Take a piece of saran wrap a little bigger than the piece of cardstock and scribble on it with the markers (or you could smoosh your stamp pads or stampin spots) being careful not to mix the colors. Then I sprayed the saran wrap with water and
carefully crumpled. Open it back up, the colors should all be swirled together and lay the cardstock face down on the colors. I used glossy white cardstock and the effect was pretty cool!
Here's one of my cards that used this technique...
Gallery at Splitcoaststampers
Hope this helps!!