Paint on one side of the paper only. Then fold it, press it flat, and open it up again — the paint prints itself onto the other half.
Paint
Brush
Way of painting
The fold makes a line of symmetry. Everything on one side lands in exactly the same place on the other, only flipped. Butterflies, leaves, faces and snowflakes are all built this way — paint close to the fold and you get a body, paint far from it and you get wings.