Designate the open area of the structure you wish to cover and create a frame using pine, fir, cedar, or any other type of structural wood. Use a either a band saw or chop saw to cut the pieces for the frame and use wood glue and nail gun to attach them to one another.
Sheath the non-window areas of your frame with plywood.
Cover the plywood sheathing with roofing paper using a staple gun to attach the paper to the plywood.
Staple aluminum can shingles on top of the roofing paper, starting at the bottom of your shell and overlaying additional layers of shingles so that when rainwater runs down your shell the water will flow off of one shingle onto another. Do not start at the top! If you do, rainwater will catch behind the lower shingles causing your shell to leak. Once the shingles have been attached, cut off the excess on the edges to create a flush edge. Caution! Shingle edges can be very sharp, so when cutting the edges wear protective gloves to prevent injury.
Once all non-window areas are covered, cut 2-liter plastic bottles to staple onto the openings for the windows.
Cover edges of windows with a thin strip of wood to increase the weather resisting ability of the structure as well as to make the structure more pleasing to the eye.