Cut a piece of string or yarn to be about 18 inches long.
Squeeze a line of paint onto a paper plate, plastic lid or aluminum foil. Make sure each paint colour connects to the next so there are no gaps.
Place the end of the string on the end of the line of paint. Then lay the string carefully on top of the entire line of paint.
Use a wooden stir stick or popsicle stick to push the string into the paint. We used a wooden stir stick that was cut in half - a clean end for each colour. (A popsicle stick would also work well).
Work your way through the line of paint, pressing the string into the paint until it's completely covered. Other than the clean end of string (you'll use that to pick up the string in a moment), you want the entire string to have paint on it.
Lay one piece of black paper on a protected surface.
Carefully lift the clean end of the string and pull the entire length of string out of the paint.
Lay the paint covered string on the piece of paper, moving it back and forth, making loops as you go. The clean end of the string should come out at the middle of the bottom of the paper. It's okay if the paint on the string doesn't reach the bottom edge of the paper.
Place the second piece of black paper on top of the first, sandwiching the painted string in between. Be sure to line up the edges.
Place a tissue box, or a small cracker box on top of the 2 pieces of paper. The tissue box might get some paint on it, so I recommend using something from the recycling bin.Place one hand on top of the tissue box. Use the other hand to slowly pull the clean end of the string. Pull the string as straight as possible, trying to keep it in the same position as you pull.
When you finish pulling the string out from between the pieces of paper, carefully pull apart the two pieces of black paper to reveal your design.
You'll have mirror images of your design on each side!
Video
Notes
If you're having trouble pulling the string, release some of the pressure from the tissue box. You really don't need to press the box down with a lot of pressure. Just press enough to keep everything from moving as you pull the string out.