I have also heard of making a base using 2 x 4's. You simply level your ground, and bolt 2 x 4's together to make a border around it.
Then, you figure out how tall you want it, and erect 'center poles' down the length of the green house.
Your next step, is to take cattle fencing, and anchor one end of it (lengthwise) to one side of the base, and the other end to the other side, this makes a half circle, with the apex of the arch resting on center pole. Repeat this all the way down, binding each fence section with wire, and to the center pole.
When your done, you pull some heavy weight plastic over the surface, and then your shade cloth.
You can erect 'ends' (Dont forget to put a door in) by either plugging with wood, or making a frame much like you just did with the cattle fencing.