There are many ways to build a raised flower bed. Most methods are relatively inexpensive; however, the cheapest way to build a raised flower bed is to use materials and components that you can source for free or as close to it as possible. Using the steps outlined below, you should be able to create a modest raised flower bed of varying sizes for much less than the cost of standard retail components.
Measure out your raised flower bed and mark the corners by hammering the garden stakes no more than 4 inches into the ground.
Cut lengths of carpentry stud to the measure of the dimensions of your flower bed. Coat the lengths generously with the all-weather sealant (it's OK to use multiple coats if you like).
Remove the stakes from the ground, attaching them to the length and width studs at 90-degree angles to form the frame of the raised flower bed. You should only need two lengths of 2-by-4-inch stud placed on top of eachother to form an 8-inch bed wall on all sides. Once the frame is complete, put it back in place, aligning the stakes at the corner of the bed with the stake holes marking the original placement of the bed.
Open several contractor bags along the side seams so that they each form a single flat sheet of heavy plastic. You should open up enough bags to sit flush against the walls and across the floor of the bed.
Staple the lead edges of the open contractor bags to the inside of the upper wall on either side of the flower bed. The plastic should overlap in the centre of the bed floor.
Fill the flower bed with potting soil.
If you are planning a raised flower bed that is larger than 2-by-3-feet, be sure that you have help assembling and placing the bed in its final position. It will be heavy. Since you are using solid wood, even smaller sizes may be too heavy to lift alone. Don't risk injury; ask for help.