All you need to do is build three or more boxes, each one slightly smaller than the other, and stack them on each other to create a unique tiered garden bed.
Depending on what you'd like to plant, 6 inches is a safe minimum depth for a raised garden bed. Keep in mind that each tier will get progressively deeper.
Cut 2-inch by 6-inch boards to the desired lengths of the boxes. Set them upright and press the ends together to create corners, using wood glue or screws to hold them together.
Place the bottom box in the desired spot and fill it with garden soil. Set the second box on top of it at an angle, creating four triangular corners for planting in the first tier.
Fill each tier with soil before adding the next, alternating each box to create four plantable corners. When done, water the entire garden bed before planting.
These easy-to-maintain beds reduce water waste, and because each tier has a different soil depth, you can grow various plants in one location without competing for root space.