Use a water-based paint like tempera or poster paint to paint the inside of the box. Paint a background for your diorama with a single shade of green or go for several shades to create depth. Paint some trees and animals, if you like.
You can paint the outside of the box as well. If your box has a glossy finish, paint it with acrylic paint.
These are trees to be placed at the corners of the box. Cut a long rectangle from brown construction paper. The rectangle's height should be at least an inch taller than the box height. Draw a vertical line ½-inch from one long side to define the side tab and a horizontal line ½-inch from the bottom short side to create the bottom tab.
Draw half of a tree trunk starting from the vertical line and branching out towards the opposite edge of the paper. Cut out the tree.
These paper tabs provide extra support for the branches and are only necessary when the leaves are too heavy for the branches.
Fold a small strip of construction paper on both ends to make a C-shaped tab. The tab must be long enough to span the gap between the branch and the box. Glue one end of the tab at the back of a branch and the other end onto the box.
Create some ferns, philodendrons, and other rainforest plants using plain paper or crepe paper. Detailed instructions and more ideas are outlined at our Diorama Plants craft page. Glue the plants to the box using a glue gun.