Choose timbers that will last well in your climate. Pressure-treated landscaping timbers are commonly available in 8-foot 5x6s, 6x6s, and 6x8s. You will need a high-quality, 3/8-inch extension bit for boring through the timbers. A chain saw will make cutting the timbers easier, but be sure to take all safety precautions. Wear gloves and eye protection when cutting pressure-treated lumber.
1. Dig a trench. Where the wood will rest on the ground, dig a level trench 2 inches wider than your timbers and about 3 inches deep.
2. Create a foundation. Pour and level a base of gravel, making it nearly as high as the surrounding grade.
3. Begin the courses. Lay the first course of timbers on the gravel. Bore 3/8-inch holes down through the timbers every 3 feet. Set precut 2-foot pieces of 3/8-inch reinforcing bar in each of the holes and pound them flush with the surface of the timber.
4. Top it off. Attach the final two courses with 12-inch spikes, predrilling 3 inches into the timbers with a 1/4-inch bit.