After laying out the marker extension cord, it's time to dig out the trench. Skim off any grass, and set the dirt of the trench aside for later use. If your wall is going to be long, a backhoe or tiller rental can be a back-saving investment, but since this wall isn't going to be all that long, the long-handled shovels and some good old-fashioned sweat get the job done.
A good rule of thumb when digging the trench is to dig it about 6" deep and fill it with about 3"-4" of fill gravel. Then tamp it down to about 2".
To determine the width of the trench, add 6" to the width of the stones you're using. This enables you to place your stones with 2" left up front and 4" left behind.