Lay your cardboard out on the ground. Draw the basic shape of your tree, including branches and fluffy top. Try to use the full length of your cardboard to make sure the tree is tall enough.
Use box cutters or scissors to cut your tree shape along the outline. With your scraps of cardboard, measure two additional strips 2 feet long and 6 inches high, and cut these out. These strips will become the stand holding up the base of the tree.
Glue the two cardboard pieces together at the top of the tree to make one solid top section. Once it's dry, pull apart the trunk of the tree, leaving a few inches between the two sections or cardboard. Cut two 6-inch slits into the base of your tree trunk, starting 2 inches from the outer edge at the bottom of the trunk and cutting upwards. You'll cut four upward slits, since you have two pieces of cardboard sides to the trunk.
Stick the 6 x 3 inch rectangle cardboard strips into the slits you just made in the bottom of your trunk. You may want to cut the strips slightly where they slide into the slits in your trunk, to make the fit more solid. Now set your tree upright, and it will rest on the base of the trunk and the two rectangle support strips.
Decorate your tree with green and brown paint--green for the leaves and brown for the trunk. When the paint dries, you can also crush up green and brown tissue paper and glue it in wads onto the tree to make it look more authentic. As you're working, step to the back of the auditorium and see how your tree is progressing from the audience's point of view. This will let you know when your tree looks stage-perfect.