Decide upon your desired shape for your tiered garden bed, such as whether you would like it to be square or rectangular. In general, square is the best option for a smaller space, as the interior beds will obviously be larger than those of a rectangular bed.
Measure the area of your base, or largest, flower bed tier. Using those measurements, determine the measurements of your next garden tier, which will be placed on top of your first one. Be sure to leave at least 1 foot of space between your first and second tiers, depending on the types of plants you will be placing there. Continue measuring for as many tiers you want on your garden bed.
Take your measurements to your local home center and have your redwood or cedar boards cut to the lengths that you need. You will need four boards per tier to create the rectangle or square that will create the box's border. The boards should be 2 inches wide and whatever length is needed for the measurements you took. Use redwood or cedar for your tiered garden bed because these materials are naturally rot- and insect-resistant. While other types of wood, such as pine, can be treated so that they are long-lasting, treated wood can release chemicals into the soil, which is may have health consequences if you plan to plant vegetables or herbs.
Assemble your first flower bed by using a drill to screw together the four boards into the shape of a square or a rectangle. Position the boards on the ground in the shape you desire. At each corner, the end of one board will be flush with the top side of another board. Use a drill to insert two screws into each corner. Drill into the outer piece of wood so that the screw ends within the interior piece of wood. Place one screw about 1/2 inch from the edge of the piece of wood and the other screw about 1 inch away from the first one.
Use a shovel to dig a trench that is about 1 inch deep around the perimeter of your proposed flower bed. The trench should match the width and measurements of the bottom tier. Make sure that it is about 2 inches wide so that the boards will fit inside it easily.
Place your bottom tier so that it rests inside the trench. This will make your tiered flower bed more stable.
Fill the tier with potting soil. If you are working in an area covered with sod or Bermuda grass, remove the existing plants before filling the area with potting soil. If not, the weeds or grass will break down on their own over time.
Create your next flower bed in the same way you made your first, by digging a trench in the center of the first bed that matches the second tier's measurements. Place the tier into the trench and fill the area with potting soil. Continue making beds until you are finished.
Place those plants with the deepest roots in your top bed so that they have the most room in which to grow.