I put the middle wall into the floor/ceiling panel, then I attached them to the outside walls.
I then attached this to the floor panel. I have put strips of wood on the underside of the base to give it a bit more strength, as it was a bit weak. Then the back panel was fixed to the back of the panels. I have glued and put more nails in the frame to ensure equal stress on each panel.
Next I fitted roof base panel. To ensure that the wall panels were straight, I lined them up with the holes in the roof panels and the walls.
This picture shows my woodworm repaired door frame panel painted.