If you take four 4x4s and put them in the ground, and four 2x4s that are 12 feet long and nail them at the top of the 4x4s, you have yourself a 12' by 12' greenhouse that will last 15 to 20 years. Tack 6 mil greenhouse plastic on the outside and you are good. If you need extra insulation, you can also tack another layer of plastic on the inside of the wood to have a 3 1/2" vapor layer. There is more to it, but that would get you started. Then, also, by adding a couple of 4x4s and three more 2x4s, you can double the size of the greenhouse when you need to. I got 15F this last winter and this is what I have used for greenhouses since 1988, but mine are a little bigger than this.