1. Use your tape measure to lay out your greenhouse on the ground using flour. Then hammer your rebar supports into the ground, four on each side of the greenhouse across from each other. Your supports should be no more than 3 feet apart. For larger greenhouses, add more rebar and PVC sections.
2. At the base of the rebar, lay out the scrap lumber into a frame and hammer the boards together at the corners. Use abutting joints, and wire the rest of the boards to the rebar supports with the heavy gauge wire.
3. Frame in the entrance end of the greenhouse using scrap lumber. Create a door frame and a window to hold the vent fan. Use boards to brace these supports. Hang the storm door in the door frame, and the vent fan in the window.
4. Bend the 20-foot sections of PVC into a horseshoe shape, and place the ends over the rebar supports to make your hoop frame. Create a ridgepole by stringing wire over the ridge of the greenhouse from one end to the other.
5. Cover the greenhouse with greenhouse plastic and tack it to the wood foundation at the bottom of the greenhouse. Cover the frame at the ends with the plastic. Wrap one end firmly and staple in place. On the entrance end, cut a hole in the plastic for your window and door. Tack the plastic to the wood around the window and door with your staple gun.