We made a dutch door for our greenhouse so it will stay a little cooler in the summer.
We'll made our door out of 2 X 4's and the same plastic you used on the rest of the building. You can miter the corners of the door but I chose to just butt them.
Carefully measure the opening for the door. Cut your frame about 1/4 inch shy all the way around. This will allow for free swinging and any wood swelling. Dado the 2 X 4's for the plastic inserts the same way we did all the window inserts from the previous steps.
Square up the frame and tie the pieces together using metal "L" brackets.
Add the hinges and hardware for the door latch. Since mine are dutch doors I added a latch between them to keep them together.
Add a strip of 1" X 2" for the door to close against. Since the greenhouse is not air tight I didn't add a strip to the top of the door or along the hinged edge.