I ordered a minimum theremin kit from Harrison Instruments and all the parts were very well marked and it went together easily. You will have to make your own antenna and put it in your own enclosure and I liked that because it gave me a chance to make my theremin unique and special. – PDF Link
There are a bunch of other kits you can buy, but this is the simplest one that I could find. Go check out a previous Make: post with lots of choices for kits. – Link
When you make your theremin, put it in an awesome enclosure and make sure to take photos and upload them to the Make: flickr pool!
You can get the pdfcast and the video podcast delivered automatically in itunes by subscribing! – Link