This is a brief guide on how I took some old windows from houses they where tearing down in my neighborhood and turned them into a small greenhouse in my back yard. I collected the windows over the course of a year and a half and the build took about 3 months, spending one day a week on it. I spent about $300 for the lumber for the frame and screws, caulk, latches, etc. That's almost 10% of what a greenhouse kit would cost. The size I built was 7 ft high x 10 ft deep x 6 ft wide. But the size of your greenhouse will depend on your windows and the time you want to put into project.
Step 1: Collect Windows and Plan Two Pair of Equal Sides.
Look for old windows and save every one you get. After you have many, lay them out and play a game trying to make two pairs of "walls" both the same height. Two to three inches won't matter as you can cover the difference with wood. Smaller holes will need to have glass cut for them or filled with something else. Keep in mind that one end will need a door and the other a hole for a fan.