Take your paper and sketch out a design. Now for my example I only used 2 colors... but you could use more. I'm hoping to show you an idea in the next few days using more color so stay tuned. Using tempera paint (water soluable) paint in all the sections you want to stay white. I used a grey shade. Don't pick a really dark color as it might leave a bit of residue /staining behind unless you are trying to achieve that look.
When it has dried take ACRYLIC paint and paint over the entire surface. Leave to dry. When dry take to the sink and wash under warm water. Whever you have painted with the tempera will lift off revealing your design in white. It also takes on this really nice weathered look due to the wash...almost batik like.
When you start washing you'll realize why you need the heavy paper....use your fingers to help scrub the paint off.
It's a really cool effect. Let dry. That's it give it a try. A very interesting technique for the kids.