Turn a large, empty shoe box onto its side. Paint the back and sides of the inside of the box green with acrylic paint to represent a forest of leaves. The top of the inside of the box should be painted blue. The bottom of the inside of the box should be painted brown to blend in with the forest floor.
Line the back inside wall and around to the sides with pieces of small-leafed plants, such as ivy or pine boughs, Place clean fish aquarium plants against the wall if no other greenery is available. Staple the greenery to the walls.
Select a twig in the yard that fits inside the box and has enough small branches to simulate a tree. Lay green lichen or moss over the branches of the tree. Many craft stores sell lichen for model trees if there is none available. Cut a section of green felt to fit the floor of the diorama. Cut a small hole in the area of the felt where the tree will poke out from. Place a small clump of clay on the floor of the box. Lay the green felt over the clay on the floor of the box. Push the bottom of the twig through the felt, into the clay. Sprinkle dirt and a few fallen twigs on the forest floor.
Cut a yellow circle of construction paper to represent the sun. Tape the sun to the outside, upper-left corner of the frame on the diorama. Stretch cotton balls to glue as clouds on the top of the inside of the box. Color the bottom of some clouds with gray or black marker. Staple several strings of small blue beads streaming down from the cloud to represent rain.
Set small, plastic animals figurines through the trees and forest floor. Include herbivores, carnivores and omnivores such as deer, rabbits, squirrel, bear, woodpeckers, hawks, snakes, mice and fox. Shape several small mushrooms out of clay. Paint the mushrooms brown or red with white spots. Place the mushrooms on the forest floor among the fallen logs.