Once the glue is dry on the fretwork panels, take the three panels and insert them into place. It should be a very snug fit. Adjust as necessary. Use a mallet to gently tap it into place and secure with wood glue. Clamp and let dry.
To build the seat, you will simply edge-join 1” x 6” boards together. Measure, mark and cut the 1” x 6” boards to length. Apply wood glue to the edges and place them edge-to-edge and clamp together until the glue is dry. Because you are joining boards long grain to long grain, you can simply use wood glue to edge-join the boards together. No additional joinery is needed here.