I took a plastic container from a frozen item, washed it, and half filled it with water. I then added drops of nail polish on top and carefully put a piece of cardstock (2 white & 2 black) on it, marbling the cardstock. You have to be quick or the nail polish will start to harden (well, it created a film that didn't transfer properly anymore).
I found that some of my nail polishes worked great for this, spreading out or streaking, while others didn't want to drip into the container at all or sank to the bottom.
Here are the 4 panels I did:
For my card, I stamped a sentiment from the Daydreamers set by Papertrey Ink in black ink into the bottom right corner. It got lost in the nail polish, so I stamped it again on a piece of vellum and folded the vellum around the back of the card to attach it to the panel. I then mounted the panel onto a blue card.