Step 30: Connect Rear Crosspiece.
Take your (1) 18.5" x 3.5" rear crosspiece and attach it to the seat stringer/rear back support as shown in the photo.
Measure one inch from the top of the seat stringer so you'll know where to attach it on the other side. Don't forget to drill, countersink, and glue. You may want to mark and trace where you want your screw holes (see picture 2 & 3)