1. Look for old windows and save them! wood frames are easiest to work with, but vinyl will work also, however they would require a frame to hold them in place. I found many on Craigslist for free, but you may want to check resale stores and recycling centers. Also you will need to have a door, you may be able to find one that has windows, but if not then you can use a solid door. *If you decide to do a solid door then you may want to place it on the side that gets the least amount of sun so that you can maximize effectiveness.
2. Clear an area to put your green house, you will want to level the ground and make sure that your surface is prepared for a long term structure. I would recommend removing the grass, bare dirt should be fine, some people would prefer to have floor, which could be created with a sheet of plywood. This is just based on your preference.
3. Layout all the windows on the ground, keep in mind that you will want to save a space for a door and ventilation, via fan placed opposite the door.
4. Once all the windows are set out how you want them, then you can determine the dimensions of your green house, each green house will be different and unique so have fun with it.
5. Building the frame. Now that you have the dimensions then you can begin frame work, 2x4s are ideal they provide a study structure and space to attach windows.
5a. Layout each side individually think about where/how you will attach the windows.
5b. Once you have the frame work done for all four sides, then you will be ready to raise the walls. *make sure that you use at least 2in screws to provide a sturdy structure.
5c. Raise one side of the frame and then brace with a 2x4, raise the 2nd, opposite wall and brace while while you raise one end piece then you can attach the end piece to both walls at the same time.
6. Once the frame is constructed then you can begin attaching the windows to the frame.
7. After the windows are attached to the frame then you can check for any gaps. If the gap is small then you will want to caulk to seal. You may want to caulk around all the windows to ensure that there are no gaps, but this can get costly.
8. The roof, there are options for the roof. One option is to construct panels like the walls and cover this frame with more windows. Another option would be to attach clear corrugated plastic panels, which will allow light in and they are relatively light and easy to work with.
9. Attach the door, you will need hinges and door knob. This step can be a bit of a process.
9a. Measure where the hinges are going to be on the door and then mark on the door frame the same measurements.
9b. Once you have the door mounted then you need to install the strike plate, this is where the door latches. Layout the position of the striker plate and mark. You will need to chisel out a place for the latch to slide in.
9c. After the space is chiseled out then attach the striker plate and check to ensure that the latch works properly. You may need to caulk around the door so there is no draft.
10. This is the time to install the fan, this is where the hole that you left open opposite the door comes into play. place the fan into the space that you left for it, you may need to caulk around the fan to ensure proper fit and draft control.
11. Enjoy your new greenhouse!