Research the Civil War. Dioramas used to teach about historical events should be based on facts. Find out some of the pivotal moments of the war that could possibly be used as your scene. Examples to consider may be the Gettysburg Address, the battle at Fort Sumter, the battle at Shiloh or the Confederate surrender.
Find pictures of Union and Confederate soldiers, especially the generals of the major battles. You can print these pictures out and create cardboard cut-outs for the people in your diorama. Having actual faces of soldiers will make your diorama more realistic. If you choose not to print out pictures, you can purchase miniature Union and Confederate action figures from a local hobby shop.
Research photographs of the scenery for your diorama. If you choose to do Gettysburg, be sure to include lots of hills and trees. To create hills you could use mounds of sculpting clay that can easily be painted and for trees you could purchase miniature trees from your local craft store.
Use a shoe box to recreate your scene. Paint the inside bottom of the box to look like the sky. You can include clouds, birds or even artillery to make it appear more realistic. This will be the background for your diorama.
Stand the box on its side so that the sky is in the background and recreate the landscape on the bottom side of the box. This will be your foreground.
Place your Union and Confederate soldiers in the foreground to simulate the battle you are representing. If you wish, you can hang cannon fire or other flying objects from the top of the box to give your diorama an even more three-dimensional feel.
Write a short summary of the Civil War event you are recreating in your diorama and tape it to the outside of the box on the back. Your summary does not have to be very long, maybe a paragraph or two, but should give enough information so that if you are not there to explain the scene the viewer should still understand what is taking place.