Drive the straight-edge spade straight into the ground, following the outline of the outside edge of the garden bed. Push the spade into the ground its full depth, holding it so the blade is vertically straight up and down.
Stand inside of the garden near the border. Hold the blade of the spade at an angle slightly smaller than 45 degrees, touching the ground about 4 to 6 inches to the inside of the outside edge of the garden. Push the blade of the spade into the soil, maintaining the narrow angle, and stopping when the edge of the spade reaches the original line, at the outer border of the garden bed.
Scoop out the soil in the resulting V-shaped trench, placing it in a wheelbarrow or bucket for use elsewhere on your property. Continue all around the border edge of the garden.
Fill the trench at the edge of the garden with gravel, pebbles, crushed shells, shredded bark or wood chips.