Step 1: Materials Needed
the wood we used was cheap, unfinished 1"x8" (nominally 1" thick by 8" wide, but actually 0.75" by 7.25") spruce typically used for decking, under finished siding or sub-flooring; for a longer-lasting - but considerably more expensive - bin, consider using cedar or pressure-treated wood (do *not* use pressure-treated wood that's older than a couple of years, as it may contain arsenic; the stuff you find in stores now does not contain arsenic and is deemed safe for projects like this).
corner posts were 4' long 2"x2"s, pressure-treated (nominally 2" , but actually about 1.5"); they probably don't need to be quite this long, but this will depend on how deep you want your bin to be and how soft your ground is.
for tools, you will need:
measuring tape and pencil;
saw (either hand saw, jig saw or table saw);
hammer and chisel (or flat blade screwdriver);
optional coffee can or similar to slip over corner posts when hammering into ground to prevent damage to top of post.
Step 2: Size Matters
from what we have read, one cubic yard/meter is ideal: any bigger and it can be hard work to turn it; any smaller and it can be hard for the pile to generate and store heat that helps in the degradation of the vegetable matter. consider the size of your household and how much you will compost. we are a household of two vegetarians, and chose the size of each bin to be 2'x2'. we already had an idea of what size we would need from our old compost heap, which was the bottom of a barrel (in both senses...), around 2' diameter. if you are very pressed for space, you could consider making the mature compartment of the bin smaller, since the compost does reduce in volume when it degrades.
in order to have an interior volume of about 2' square, we made the overall length of the long sides:
24" bin 1 +24" bin 2 + 2" end 1 overhang + 2" end 2 overhang + 1" notch + 1" notch +1" notch = 55"
and the length of the short sides:
24" bin + 2" end overhang 1 + 2" end 2 overhang + 1" notch + 1" notch = 30"
we have a small car, so ended up having the lumber yard cut the lengths on site for us. we also bought precut corner posts (typically used for deck railing balusters, hence the angle cut on end) which were ready to be hammered into ground as-is.
so, to make a bin this size you will need:
4 x 55" lengths;
6 x 30" lengths;
4 x corner posts.
our total cost was less than $15.
Step 3: Location Location Location
you obviously want the ground to be level, so we filled it in a little where needed and tamped it flat.
Step 4: Cutting to Size
measure in 2" and 2.75" from each end of all boards; find the center of the long boards and measure a 0.75" wide channel there; then measure a depth of 3.625" (half of the 7.25" depth of the board) on each notch. the shaded area in the fourth picture is what you end up cutting out.
we have a table saw, so it was quick and easy to make the two vertical 3.625" cuts, but a jigsaw or handsaw works fine too. you can snap off the remaining notch and might get lucky, but will probably have to clean up the cut with a chisel (or flat blade screwdriver) anyway.
note: err on the side of making your notches slightly too big rather than slightly too small - it will make assembly easier and gives some wiggle room.
Step 5: General Assembly
check the positioning on the ground (and fill in depressions/level off humps in ground, if necessary).
take your first corner post and hammer into ground (start in the least accessible corner and work your way out so that you have less hurdles to deal with). we used a coffee can to protect the top of the post from splitting after being hit with the hammer, but you can also use an old piece of 2"x4" or flat metal bar. repeat for the other three posts.
add the second course of boards as per the first.
Step 6: Fill 'er Up
it's good to have a bed of leaves to start with, for both the matured compost and the fresh pile you are about to start. luckily for us, it's fall in new england, so there's no shortage. chuck in any worms you can find too - they always help break down the vegetable matter.
we added our old mature compost to one side and covered with leaves (will help to insulate over the winter, though this isn't essential) and fresh kitchen waste to the other.
the compost on the left will be ready for use next spring when we start planting the garden. once we've emptied that, we then start filling it with fresh waste while the other side matures for a year. and so on, and so on.
update 10/11/08: added a pic to show how it looks now. it has weathered pretty well, and has stood up to having been moved once. should be a good few years left in her yet.
update 04/19/09: added a pic of the first fully developed 1-2 year old compost: really nice and loamy, clean and smell-free. despite this picture being taken only two weeks after the last of the snow had melted, the compost was very dry and crumbly, not a soggy wet mess. we used the blue tray to screen out the larger pieces (mostly twigs, walnut shells, peach pits and the like).
update 08/15/12: we replaced all the wood and built a new one. in truth, the pressure treated corner posts didn't really need replaced, but we had some new cedar ones lying around anyway, so why not. the biggest problem wasn't that the wood on the sides had rotted as much as most of the "tongues had got knocked off the ends of the boards over the years for one reason or another. if they had some kind of metal plate reinforcement, you'd probably get closer to ten years out of these basic pine boards.
Step 7: Possible Mods
you can use wider boards if you can find them; ideally, we would have used two courses of 1"x10" boards, but could only find 8" wide. in the end, they turned out to be perfectly adequate.
we will update this instructable over the years to see how the wood holds up. we don't have great hopes for it lasting much more than 5 years or so, but still consider it a worthwhile investment for $15. pressure-treated wood would have cost around $50, and cedar likewise .
if you are building a bin that is much bigger than this, it might be better to use slightly thicker boards for extra support. these cheap spruce boards might not be effective at standing up to the pressure of a bigger pile and the forces exerted on them when you're turning the compost. thicker corner posts would also help in this case.
disassembly is super easy (the opposite of assembly, but you don't need to remove the corner posts), and it might make sense to just take it apart come spring when you want to shovel out the old compost. also, you can move the whole compost bin to a different location relatively easily any time you want.
a triple or quadruple bin is possible. if you live in a temperate region where you can grow (i.e., use your compost) year-round, and have a good decomposition rate for your compost, this could make sense.
we deliberately left our corner posts long in case we have the need to add a third course of boards. if there wasn't a possibility of that happening, we'd probably cut them down to size. besides, they could be useful for hanging things on.
we would have loved to have used old wood we had lying around for this project, but alas, all we had was painted (and probably containing lead, at that) boards, which we deemed unsuitable.