Building raised beds is an excellent way to set up a garden, especially if your ground soil is not very suitable. You will have far fewer weed problems to deal with; the garden will look more organized; and the plants will be easier to access. You also have the ability to completely customize the soil for whatever plants you intend to grow.
Determine the location and size of the bed you want to build. Look for a place with the right amount of sunlight for the plants you want to plant. When considering what size to make your garden bed, keep in mind that you will want to be able to reach the middle from at least two sides.
Purchase boards of the desired length and width. While you are at the lumber store, ask an employee to cut the 4-by-4s into six 12-inch sections. This will save you a little work later on; if the lumber store will not do it, you will have to cut them yourself. Cedar is the best material for boards because it resists rot very well.
Apply waterproofing sealant to the wood, using the paintbrush. Depending on the type you use, this will give the wood a nice appearance as well as protect it from all the water you will be putting into the soil. Allow the wood to dry completely.
Set up one of the corner posts (a 12-inch section of 4-by-4). Take two shorter 2-by-6 boards and place them flush against the corner post, one on top of the other so they stand 12 inches high. Drill pilot holes through the 2-by-6 boards into the corner post, making two holes for each board--a total of four. Drill in the screws.
Repeat the same process for the other side of the 2-by-6 boards, and then again for the second set of shorter 2-by-6 boards.
Place the first set of two longer 2-by-6 boards against one side of one of the assemblies you created in the previous step. The end of the longer boards should be flush against the surface of the shorter boards. Drill four pilot holes into the corner post and insert screws.
Repeat this step for the other end of the first set of longer boards and for the second set of longer boards. At this point, you should have a complete box of boards. Mount the final pair of 4-by-4 posts in the middle of the longer sides of the box. This will help the box to retain its shape and resist warping.
Place the box in its desired final location. Cover the ground with landscaping cloth or something similar. Fill the box with your desired soil mix. Consider adding worms to help maintain the soil quality.