Here are the quick and easy steps:
Cut out the shape you want (or just use a rectangle) and tape it to the bottom of the cardboard box or plastic tupperware tub.
Have your child add paint in the desired colors and areas.
Add marbles! Have your child tip the box from side to side to roll the marbles through the paint.
When you are happy with the finished product, take it out to dry. I glued my sons to another piece of paper so we could use it as a table centerpiece.