The original plans are using 1x4 lumber. However, in Canada, you really want to upgrade to 2x4 trusses. Snow can be pretty heavy.

Start by connecting 2 PVC pipes together using a 45 degree connector. If you don't want to glue the pipes so that you can dismantle it later, use round quick pins to attach them.

Temporarily attach both ends of the assembly to each side of a 11 feet 2 by 4. This gives you a rough idea of the dimension of the side of your greenhouse.

You can now decide what size your door should be. Mine is 3 feet but you can make it larger if you want to drive a large wheelbarrow throught it. The height is also up to you.

Now that you have decided the width and height of your door, you can measure and cut all the other pieces of wood (cf drawing).

Cut some plywood, apply some glue and drill it onto the 2 by 4 boards. It will keep everything strong and square.

Drill holes in the PVC pipe in the 4 locations mentioned on the drawing and use tie-wraps to keep it even stronger.

You can now staple plastic on each sides. Cut a hole in one of them for the door.