Check with your community's building department before setting out to build a retaining wall. Many codes require a permit for any structure that holds back what amounts to thousands of pounds of earth, and most limit the height of an amateur-built retaining wall to 3 feet. If your slope needs a higher wall or requires extensive grading, call in a masonry or landscape contractor -- or terrace the slope with two or more lower retaining walls.
In addition to the usual carpentry and digging tools, you will want a baby sledge for hammering the 12-inch spikes. (If you have trouble driving these without bending them, consider predrilling holes.) Consider renting a chain saw: Cutting 6x6s with a circular saw requires several passes. In addition, a chain saw will let you trim the timbers in place. If your slope is extremely irregular or large areas need cutting, consider hiring an earthmover. Be sure to install the drainage gravel and pipe as indicated, or water pressure (a massive amount can build up) will eventually cause the wall to buckle.
1. Plan your wall. Plan how your retaining wall will fit together, particularly the locations of the deadmen along the third course. Dig back any irregularities on the slope, allowing for at least 8 inches of backfill. Trench T-shaped cavities for the deadmen.
2. Prepare trenches. Excavate a level trench that is 9 inches wide and an average of 6 inches deep. If necessary, dig behind the trench, so that there will be at least 8 inches for the drainpipe and gravel. Dig trenches for the deadmen. Spread 2 inches of gravel (more if you have soggy conditions) in the bottom of the trench.
3. Lay the first course of timbers. These and all the other timbers should be level along their length, but should have a 1/4-inch pitch to lean the wall into the hill. Apply construction adhesive between courses for added bonding and to keep water from seeping through them. Add the second course, attaching it with spikes every 3 to 4 feet. With the third course, install deadmen with cross ties.
4. Finish. Install your remaining pieces, and provide drainage. Install the drainpipe (pitched 1/8 inch per foot) on the gravel bed. Backfill with gravel up to the top of the second course. Cover the gravel with filter fabric (or tar paper) and finish backfilling with soil.