Use a large sheet of construction paper or a large roll of banner paper. Put a small amount of paint on a paper plate. Gather the Duplo bloc sizes you want to use for stamping.
Use the blocks to stamp in the paint and then onto the paper. In the hope of making it less messy, I gave each child 2 blocks to use for each color of paint.
One of the blocks was used to stamp the rectangle or square. The other block was used to stamp the circles in the middle of the rectangle or square. If you don't mind a messy paint experience, then you can dive in and use one block: stamp with the flat side for the rectangle and flip it over to stamp the circles.