How do I assemble the back support for my Graco booster seat?

Flip the head support and lower back support over, then squeeze the red adjustment button and slide the two pieces together until they snap into place. Attach the elastic strap, insert the fabric from the head support underneath the fabric on the lower back support, and then attach the backrest to the base by interlocking the U-shaped hooks on the bottom of the backrest with the bars on the base.