See a video tutorial for Step 12 and/or follow the instructions below:
For the window frames, line up the ventilating window horizontal (part 19) and vertical (part 20) pieces and screw them together. Hinges are attached to the preferred end of the frame.
For the doors, use the two vertical (part 12) pieces and 3 pieces of door diagonal strap (part 15). Screw these pieces together with parts 15 going tying up at both ends and the middle (use the internal pair of holes).
Use a pair of parts 14 and 15 as diagonals for the door. Part 14 goes inside whereas part 15 will run diagonally on top of part 14.