Measure the area where you plan to place the wall with a measuring tape. Sketch out a simple design for your wall using the measurement. Stagger timber joints as you draw, and include 8-inch wide areas for deadheads, or supports, starting at the third row of timbers. Use the sketch to help you figure out how many pieces of 8-by-8 foot pressure-treated lumber you'll need to complete the job.
Dig a trench where you want your wall to be. Make the trench at least 16 inches wide and deep enough to sit below the frostline in your area. Although the wall does not have to start below the frost line, it typically makes the wall more stable and decreases the chances of long-term problems, like shifting timbers.
Fill the trench with 4 inches of gravel. Tamp it down with a tamper or the end of an 8-by-8 timber until it doesn't shift. Add 4 more inches. Repeat the process until the trench is filled with gravel.
Cut the timbers to the appropriate length with a circular saw. Use your sketch as a guide for the length of each timber.
Measure the diameter of your rebar, if you don't already know its diameter. Drill holes every 4 feet into the timbers intended for the first row using a heavy duty drill and a spade bit the same diameter as the rebar.
Place the first row of timbers down along the center of the gravel. The end of one timber should sit flush against the end of the next timber.
Use a sledgehammer to pound the rebar into the ground through the holes in the timbers until the rebar is flush and even with the top of the row. Use a reciprocating saw with a metal-cutting blade to trim the rebar even with the timbers, if needed.
Place a length of perforated pipe, equal in length to your wall, behind the first row of timbers. Make sure the perforated holes in the pipes are facing toward the ground to increase water flow and drainage.
Lay the second course.
Place 3 timbers perpendicular to the wall--one at either end and one at the center of the wall. One end of each timber should sit on the wall, flush with the front of the wall. Bury the other end of each timber in the soil behind the wall to add stability and support to your wall. Cut timbers with your circular saw to fill in the gaps in the wall between the ends of the perpendicular timbers to finish the course.
Lay the next course of timbers.
Add two more supports, staggered between the first three, to the next course. Use the circular saw to cut timbers to fill the gaps between the supports. Finish laying the course. Lay another regular course. Finish building the wall.
Use a heavy-duty drill to deep drill holes 8 to 10 inches from the timber joints at the top of the wall and above your supports. Drill all the way down until you reach the ground. Do not drill through the old rebar in the first row.
Cut pieces of rebar to the proper length. Make pieces to go through the supports at least 2 feet long. Make all other pieces at least 2 feet longer than the height of the wall. Slip the rebar into the correct corresponding holes. Use a sledge hammer to drive the rebar into place. Cut off any excess using a metal-cutting blade on a reciprocating saw.