Mark out the planned location of the retaining wall around the garden with brightly colored spray paint.
Use a shovel to dig a trench for the retaining wall. The trench should be deep enough for you to bury 1 inch of slate blocks for every 8 inches of the total height of the retaining wall.
Apply a 4-inch layer of crushed stone and compact it with a hand tamper.
Add 1 inch of coarse sand to the crushed stone. Compact again.
Lay the slate blocks in place atop the coarse sand. Lay them as closely together as possible.
Pour crushed stone over the first row. The crushed stone goes between the joints to fill them in.
Continue adding rows of slate blocks and pouring on crushed stone. Use a level and string to ensure the rows and columns are even before adding additional rows.