Stairs usually have two parts; a horizontal piece you step on called a tread and a vertical piece at the back of the tread called a riser. On our stairs, they were made from "Two-by-Six" lumber and each were around four feet long. There's no magic to the size, but our beds came out slightly rectangular because the treads and risers were different links. It doesn't matter. If you have to buy lumber, you can use all four foot long pieces.
A note about lumber: When you go to buy wood, it's usually labeled "two-by-four," "four-by-four" and so forth. These are supposedly the size before finishing. Whatever the reason, lumber is ALWAYS smaller than the numbers suggest. In this project a "two-by-six" measures about 5-3/8 by 1-1/2, while a "four-by-four" post is about 3-3/8" square. Don't worry about it.
Each side of the bed is made of two stacked pieces of two-by-six about four feet long, so eight are needed for each box. The sides are braced by one-foot-long pieces of four-by-four at each corner, so four of those are needed for each box.
This drawing and parts list shows how the beds go together:
For two boxes, I bought an eight-foot-long piece of four-by-four and sliced it into eight one-foot lengths.
If you were buying wood for the sides, you could use eight four-foot lengths of two-by-six, cut from two boards each eight feet long. If you can, get the store to do the cutting for you. I cut the corner posts myself using a hand-held circular saw, which is convenient but very dangerous. If in doubt, use a hand saw instead.
Each of the eight side pieces is screwed to the corner post with two lag screws. The ones I used are 3/8 inch in diameter by three inches long. I used a 3/8 inch flat washer under each screw head.
Lag screws have a hexagonal head. The 3/8 inch size can be tightened with a 9/16 inch wrench.
As you can see, the screws have a large smooth section, called the shank between the head and threaded portion. When you install a lag screw, you drill a hole for it with an electric drill. Holding the side against the corner drill a 5/16 inch hole through the side and into the corner post. Make the hole about three inches deep. This pilot hole keeps the screw straight. It's slightly smaller than the threaded part of the screw, so the threads can bite into the wood. After the pilot hole is drilled, you drill a second, larger hole through the side piece but NOT into the corner post. This shank clearance hole is 3/8 inch in diameter. The screw should tap in easily through the side piece, then you tighten it down using the wrench. I usually leave each screw slightly loose, about 1/8 inch above the side piece, until all four screws on that side of the box are in. Then I tighten each of them until the washer is pressed slightly into the wood. If you over-tighten the screw, you can strip the hole, so the threads can't bite and the screw turns freely. If this happens, just drill new pilot and shank-clearance holes nearby and try again. The exact position of the screws is not at all critical. On the drawing, you'll see that I offset the screws on adjacent sides a bit so they won't hit. This isn't likely to be a problem anyway, though.
To start the assembly, lay two corner post pieces about four feet apart. Lay two sides on the corner posts. Using lag screws and washers, attach the sides to the corner post. Make two of these assemblies.
Notice that the corner posts stick up a bit above the sides, because the two sides add up to less than 12 inches. The end that sticks up will become the top of the bed and the other end the bottom so the bed will sit flat.
Using four more side boards, attach the corner posts together using lag screws:
That's the end of the carpentry! Now turn the bed over so the flat bottom is facing up. Using a staple gun, fasten weed-block fabric to the base of the bed:
The fabric I used came in a three-foot-wide roll, so I overlapped it in the middle, leaving a few inches of margin beyond the frame, then fasten it down with staples.
To keep gophers and other critters from burrowing into the bed, I covered the bottom with screen, or, as the hardware-store people call it hardware cloth. Made of galvanized steel wire, the size I used had quarter-inch mesh and came in a four-foot wide roll. This is an expensive item -- I paid around $5 a foot for it, and you need 4 and a half feet for each box. But I think you'll regret it later if you do without.
Leaving an inch or so of overlap on each side, staple the hardware cloth to the frame on top of the weed-block fabric. If you bought it in a long roll, as I did, cut it to length using tin snips.
Fold over the excess fabric and hardware cloth and staple them to the sides. Assembly is finished!
This "see-through" view shows how the fabric was overlapped in the middle:
You can stop here. I decided to stain the wood with a brown transparent wood stain to even-out the color, especially of the raw wood at the ends of the corner posts where they were sawed.