Assemble the Frame and Add the Sides
For the base frame, cut four pieces of the 2x3 material at 43" and four pieces at 32" for the horizontal bottom and top of the frame. Cut four pieces at 24" for the uprights of the frame.
Using decking screws, assemble the bottom frame and top frame and then attach the corner posts between them.
For the roof frame, miter cut four pieces for the corners, the length will depend on the desired roof pitch. Cut three pieces to span the four corner pieces. Place one at the roof's peak and the other two centered on the sides (Image 1).
Measure and cut the plywood for the sides, front and back (Image 2). Attach using decking screws.