The process is straightforward, but the timbers are heavy, so making a landscape timber box takes physical strength. Timbers come in 8-foot lengths and 5x6-, 6x6- and 6x8-inch dimensions. Pressure treated timbers and railroad ties are commonly used. If you don't intend to make a box that's 8 feet long on each side, you'll need a chain saw to cut the timbers. Plan on spending a weekend building this box and enlist some helpers.
Use a string and stakes to create the outline of the future box.
Dig trenches along the string line 3 inches deep and 2 inches wider than the landscape timbers you'll use. Level the bottom of the trenches.
Pour gravel into the trenches to about ½ inch from the top of the trench. Smooth it out.
Using a 3/8-inch extension bit on a drill, bore holes through four of the timbers at about three-foot intervals.
Lay the first row of timbers in the trenches, creating the bottom of the box.
Place a 2-foot-long reinforcing bar into each hole and pound them into the ground until the top of the bar is flush with the top of the timber.
Make 3-inch pilot holes in the ends of the remaining timbers with a ¼-inch bit.
Set a timber on top of the first course (or layer) of timbers and drive a 12-inch spike through the pilot hole and into the timber beneath it.
Continue until you've built a box that's three timbers tall on each side.