I think the easiest way would be to remove all the panels, vents and the plastic on the other half of the roof. Then get a piece of greenhouse plastic that will fit over your entire roof (measure the length of the greenhouse x the length of each roof half from peak to where the roof meets the wall) make sure to leave at least a few inches excess on your plastic to give you room to adjust. Then on a NOT WINDY day, get a friend or 2 to help place the plastic: use a broom or stand on a ladder to get your front edge up on top of the roof from one of the ends. Drape it down the sides so that each of you can grasp an edge from either side. Pull your plastic over to the other end, like making a bed. You might need a third friend to stand inside the greenhouse to poke it up with a broom to keep it from getting snagged. Then staple all the edges to the frame. You do not need to staple anywhere else. After, you will want to nail thin strips of wood over your edges to prevent the wind from catching the plastic and tearing it through the staples. Done! This is how I have done my greenhouse roofs. Good luck.
