When I needed a greenscreen to try some chromakeying, I went with the lazy and more expensive approach of buying something for around $150 through a theatrical supply store. Not the same as doing it yourself, but the timesaving tradeoff looked better under the circumstances. If you're on a seriously low budget, consider the DIY solution detailed at Jeff's Computer Support Online. Jeff provides step-by-step instructions on constructing a greenscreen from muslin fabric and PVC piping. The upside is a lower cost of materials for your project. The downside to this approach, as Jeff points out, is that the PVC frame isn't very mobile when put together. Apart, the frame is lightweight and could easily be transported anywhere.