Lay two 4-foot-long and two 8-foot-long landscape timbers over the site to form a rectangle that defines the outline of the raised bed. Align the corners of the lengths to lie flush against each other. Insert a shovel through the turf to mark the outer and inner perimeters along the four lengths. Remove the timbers and place them aside.
Remove the grass between the outer and inner perimeters with a spade. Use a trenching shovel and dig further to a depth of 5 or 6 inches to make a trench for the walls of the raised bed. Collect the dirt in a wheelbarrow as you dig. Once complete, tamp the base of the trench to level it.
Spread a 2-inch-thick layer of gravel into the trench with a hoe.
Lay two 4-foot-long and two 8-foot-long timbers into the trench. Arrange the landscape timbers so the end of one length joins the other in a clockwise pattern.
Drill 1/2-inch-wide pilot holes every 2 feet into the four landscape timbers. Insert a 1-foot long rebar into a hole and pound it with a sledgehammer until it penetrates the soil below and its top lies in level with the surrounding timber. Repeat the process to pound rebars into all the pilot holes.
Place two more 4-foot-long and two 8-foot-long landscape timbers directly above the previous ones, but in an anti-clockwise pattern. Drill five timber screws through each length of landscape timber -- two near each end and one in the center -- to join it to the one below.
Drill 1/2-inch-wide drainage holes through the sides of the newly placed timbers. Drill one hole through each 4-foot length and two through each 8-foot-long landscape timber.
Place the last pair of 4-foot-long and 8-foot-long landscape timbers directly above the previous ones, in a clockwise pattern. Secure each length to the one directly below by drilling one timber screw in the center and two at each corner.