Assemble the empty boxes and tape them shut with packing tape.
Lay newspaper to protect the floor. Place the boxes on the paper and paint them on all sides, except the bottom, with white primer. Allow the first coat to dry, then add a second coat, repeating as necessary until the boxes are opaque white with no box graphics showing through. Allow the boxes to dry for a day.
Sketch the scenery on the boxes with a pencil, stacking the boxes if necessary. For example, for a tree, stack three painted boxes and sketch the outline vertically. For a bush, use one box. Think of the boxes as paintable building blocks, but don't stack more than three high.
Hot glue the stacked boxes together to prevent them from toppling.
Paint the sketched designs with acrylic paints. For plays with changing scenery, sketch and paint a different object on different sides of the box, to be turned as necessary to set the scene. For example, if the play takes place in different seasons, one side may be painted with a summer tree, another with an autumn tree and another with a tree in winter.
Allow the scenery to dry for at least 24 hours before using it.