Powered by just a little tension energy, the rubber band balsa airplane is an experiment that can teach any kid the fundamentals of flight.
Buy a propeller. This is the only part that you are unlikely to find easily. (See Resources.)
Cut the main body of your airplane out of balsa wood. The design should be about 2 inches high, with a top fin for stabilization. Basically, this piece should be all the vertical components of your final craft. It needs to be long enough to accommodate the main and back wings, but allow for a great deal of customization beyond that. Your fin should rise out of the farthest back portion of the body, and it should be at least 1 1/2 inches taller than the body's height.
Cut the two wings out of balsa wood. Design your wings to cant back toward the plane's aft. Think of a 747's wing design (or a boomerang) in making your own. The most important part of both wings is to ensure that the center of the wing is the thickest part. Construct the back wing to be the same shape as your main wing, just 1/4 the size.
Cut two horizontal slots in the body of your airplane. These slots hold your wings in place, so make sure they are of the same length as your wing's maximum width. The wings should center and hold firmly in place. The full body of your craft should be evident at this point.
Install your paperclip engine mounting. Affix your paperclip to the underside of your plane, immediately beneath the tail fin. You can use superglue, or puncture the body of your plane and wire one end of the paperclip through it. The final point is to have an eyelet that can be twisted open or shut on the immediate underside of your plane.
Mount the propeller. The propeller is designed to slide onto the rounded balsa nose of your plane. Permanently affix it with a single dab of glue. The wire eyelet of the propeller should run out of the bottom to the underside of your plane's body.
Build the engine. Run a single rubber band from the eyelet on the propeller to the homemade eyelet on your paperclip rear mounting. Twist both eyelets shut. The ideal rubber band is thick enough to withstand multiple uses and of a length to hold tautly to the underside of your plane even when no pressure is being exerted on it.
Twist the propeller with a finger, then gently toss the plane. Any terrible deviations in its flight pattern, particularly loops, may indicate improper weighting. Gluing pennies to the front of the plane, just along the propeller, can help weigh down the nose and steer her true.
Watch your fingers, as rubber band pressure can spin the propeller faster than you might think.