Update: Due to popular demand for information about how to care for worm bins, I'm thinking about writing a small book about the subject. Please let me know if you'd be interested in having a little worm bin manual! (Yes, I know there is another such book. I'd like to do one in a slightly different style.)
Years ago, when I was poor and under-employed, I craved a worm bin (aka vermicomposter, aka worm composter), famous for the fastest compost in the West. I did research on the web, and found that commercial bins were expensive, as much as US$200 for an Australian multi-tray "worm farm", which was way too big for my apartment-dwelling self anyway.
That winter, I visited my sister in Oregon nursery country, and she had the brilliant idea to use nursery flats as trays. Guess what some of the commercial bins are made from? One U.S.-made bin got started that way, and has since done some customizing, using their own molds.
I've seen simpler versions of worm bins, a 5 gallon bucket, or a stack big Rubbermaid tubs with a lid. They probably work as well, at least until you want to harvest your worm castings, which you must sift out of the newer bedding and food scraps; not to mention fill a perfectly good tub with a ragged pattern of holes.
The tray version seen here allows you to segregate old from new, in just a few minutes. It also makes trips between "floors" much shorter for the worms. Less crawling, more eating. And pooping. Worm poop is good.
Mine has a couple of issues I have not gotten around to solving, more on that in the last couple of steps.
Update, Sept. '07: After all these years, I finally realized how easy it would be to separate the liquid from the food and castings. The castings I had been getting were thick mud.
Enter the filter! I lined the next to bottom tray with heavy shade cloth, usually used overhead for shading plants, etc. You'll see it in the shade plant section of the nursery where you go to get your flats. I'll post photos later.
Onward to the building part...
Update, : May 28, 2008 See step 7 for some info on how I harvest the castings.
Update March '09: There seems to be a steady stream of questions about how to maintain worm bins. People seem to want more detail than I have provided here, so I'm thinking about writing a small book.
Update March '12: No book has been written yet, but I'm seriously considering making an ebook/iBook. If you're interested, please let me know, it will help me determine the interest level.
Please let me know, preferably via comment or private message, what delivery method you'd prefer. Paper book?, e-book? I kind of like the image of an intrepid composter outside, muddy hands clutching a Kindle. ;-)