Cut one 4-by-8-foot piece of cedar in half, making two approximately 3 1/2-foot-long pieces of wood. These will be the two shortest ends of your raised garden bed.
Build a box with the two 3 1/2-foot pieces of wood and two additional 4-by-8-foot pieces. Use a drill to overlap the edges and secure them into place with 3-inch deck screws. You will end up with a box approximately 3 1/2 feet wide, 7 feet long and 3 1/2 feet deep.
Cut one piece of 2-by-8-foot lumber into four equal pieces, approximately 1 3/4-foot each. Secure one piece to each corner of the box diagonally with 3-inch deck screws. This will help to brace the corners, preventing the sides of the box from bowing. Most raised garden beds are staked to the ground, making this step unnecessary. It is usually not possible, however, to stake the bed into concrete.
Place the box in a sunny location on the concrete area. Put down a layer of crushed rock or pea gravel so it evenly covers the bottom of the raised garden bed. This will allow for adequate drainage, protecting your plants from sitting in too much water.
Measure the inside dimensions of your box, including the length, width and depth. Multiply these three numbers to determine the cubic feet of your box. Then, purchase enough garden soil to fill the area. Beds that are placed over dirt or grass are typically much shorter than the 3 1/2-foot-high box described here, thus requiring significantly less dirt to fill it. This is because while you can build a raised garden bed over concrete, the standard 1- to 2-foot-deep garden bed will not provide the space that most plants need when installed over concrete. In most cases, the garden soil will be the most expensive part of this project.
Fill the box with garden soil that is mixed with compost or fertilizer.