I havn't done this much, so I'm still working on making it look better. I did it once before to make an artist palette out of thin white foam, and just recently for the christmas cards I'm making this year.
Gather together what you'll paint, tape if you want clean edges, and a sponge to paint with. I prefer a sponge for each color so I cut the one I have.
When you're ready to paint, start with your lightest color paint and wait for that to dry before you continue.
When it's dry, use the next color, streak the sponge quickly back and forth in a straight line.
Finally use your darkest color and streak that back and forth in a straight line.
If you want, use the dark color again to make more of a wood look. Finally when you're ready peel of the tape if you used it.