Stand two corner posts up on flat ground. Slide the first long board with bottom support slat down through the two posts. The dovetail attachments might be tight. It is easiest if posts are directly vertical.
Repeat with second wall of bed. Add middle narrow board to either side. Add third board to both walls (this board does not have support slat).
Next, add soil cloth and 4 corner post caps. If you are adding a drip irrigation system, you would add that here, trailing the tubing up the inside of one corner post to hide it.