How to build a one-piece
folding picnic table out of 2x4 lumber
Step 10. Add the seat boards
Place the seat boards in place on the side-frames. It should be obvious which boards go where.
The shorter seat boards [l] fit in between pieces [f1] with the back edge flush with the back of piece [g] and the longer seat boards sit flush with the front and ends of the frame.
Fix the front boards first and then the back boards ensuring everything is square and plumb . Fix with screws. At this stage do not use glue in case any adjustments are needed. You can unscrew, glue, and re screw at a later time when you are sure the mechanism is working properly.