First off, you start with painting your nails with the base colour. In my case, it was medium blue - 875 It's All I Can Blue by Catrice. Then you wait until it's dry. Now you need to scrunch the saran wrap to a ball, and place it in your reach. Take the second nail polish you would like to add, and just paint one (rather thick) layer on top of the base colour. Don't wait but immediately take the scrunched saran wrap and dab around on your nails. The saran wrap will randomly take off bits of the top nail polish and will make the base colour shine through again on some spots. The longer you dab on your nails, the more of the top nail polish colour will be taken off, which means that the base colour will be more visible. Now that you know, you can play around with this unique pattern this technique creates. I think it looks awesome!
One hint: If you mask the skin around your nails with scotch tape, you won't have to clean up the mess after using the saran wrap.