Last step we put down a foundation and I recommended 6" X 6" landscape timbers. It's important that they are in the ground good a solid and level.
This step we will start building the frame. The frame is nothing more than a simple 2" X 4" stud frame. I used treated wood on the bottom and cedar for the rest. Cedar is a little spendy but I like the way it ages without a finish and I like the smell. Do your framing on a flat surface and be sure to square it up before nailing.
The shape of your frame will depend on your design. Since I don't have a good picture of my greenhouse when it was framed, here is a doctored picture to give you an idea.
Here is another doctored picture from the front to give you a little better idea of the framing. I've whited out the inset panels to just show the frame.