Hammer the rebar 6 inches into the ground in two rows of three stakes each. Each stake should be about 5 feet away from the others in both directions. Make sure that the rebar is securely placed in the ground and cannot wiggle back and forth. You may have to tap it further into the ground.
Place the end of one of the PVC pipes over one of the stakes. Bend the pipe over until it can be slipped over the stake directly across from it in the other row. Repeat this process three times until you have a straight line of three bent tubes resembling a low tunnel.
Stretch clear plastic sheeting over the greenhouse frame with plenty of extra overlapping at each end to be formed into the doors. Secure the plastic sheeting to the frame by poking small holes in the plastic on each side of the frame and connecting them together using a zip tie wrapped around the frame. Before poking the holes, place a strip of clear packing tape over the area to strengthen it, and apply additional tape after the zip ties have been added to hold the entire configuration in place.
Stake the plastic sheeting to the ground if necessary, or attach it to the rebar at the base of the PVC pipe for extra stability.