Figure out what size raised bed will work best in your garden. Larger raised beds, those that are more than 4-square feet, will need stronger support at the corners than smaller beds, which will be fine with inexpensive L-brackets from the hardware store.
Overlap the short wall of your small raised bed over the ends of your longer walls. Drill guide holes through the overlapping wall directly into the ends of the longer walls, and screw the walls together.
Reinforce the corners by attaching 2-inch L-brackets in the inside top and bottom of each corner, making sure to stagger the brackets in between the main screws.
Cut 4 lengths of 4-by-4 lumber, with each piece at least 4 inches longer than the height of your raised bed.
Drill guide holes and screw the long walls of your bed to the posts. Leave the extra 4 inches of post exposed on top of the bed until you have it built and can dig it in and flip it over.
Level the bottom edge of the sides and screw the short walls of your bed to the posts instead of into the other walls.