As each tray fills with castings, you can repeat the process in the tray above. Worms will migrate through the mesh to the new source of food, and you can then spread the contents of the lower tray on your garden or greenhouse beds or use it to enrich potting soil. Put the newly emptied tray at the top of the bin, and the cycle can go on indefinitely. Another advantage of commercial multi-tray bins is that they're portable, so you can set one up outdoors in a shaded area during the growing season, then move it to the greenhouse, sunroom, or basement when cold weather arrives and continue composting.
Homemade bins. Commercial bins work beautifully, but tend to be pricey. If you don't want to pay big bucks for a worm bin, you can make your own from a plastic storage bin, such as a 3 feet by 2 feet by 1 1/2-feet-deep storage bin, a modified garbage can, washtub, or wooden box. Use an awl to punch small holes in the sides of plastic washtubs or garbage cans for aeration. To keep conditions moist but well drained, make a drainage area in the bottom of the bin; use a rigid divider to separate it from the worms' living quarters. A loose cover keeps flies and light out and worms and moisture in.
Just as with commercial bins, it's best to fill the bin with soaked coir and newspaper. (You can buy compressed coir bricks from garden supply stores, catalogs, and Web sites, and from sources of worm-composting supplies.) Garden soil may also be added. Make sure the mix is as damp as a wrung-out sponge rather than wet. Then introduce the purchased earthworms to their new home. Use your purchased worms for composting only—most commercially available worms are species that live only in manure or very rich soil and will not survive in average garden soil. One exception is the enriched soil in a greenhouse bed—if the greenhouse stays above freezing, worms will do very well there.
Feed your worms well-chopped vegetable matter mixed with a bit of water. Soft foods are best for the first few days; if food doesn't disappear in 24 hours, reduce the amount. For faster composting, run the food through a blender, since worms don't have teeth to tear off large chunks. The population should double in about a month; after 60 days, your bin should be full of rich compost.
To harvest the compost, but save your earthworms for another session, place the compost outdoors on a sheet of heavy plastic or fabric, and let it sit for about an hour. The worms will cluster together to stay cool and moist. Dig in and find the cluster. Return the worms to the bin or start a second bin.