When and how should I use the shoulder belt positioning clip?

If the shoulder belt lays outside the red zone on the child's shoulders, you must use the shoulder belt positioning clip. Insert the loop end of the strap around the vertical bar on the rear of the seat (closest to where the vehicle shoulder belt crosses the child), pass the clip and strap through the loop end, and slide the vehicle shoulder belt onto the clip. Pull the strap down to tighten the clip against the child's shoulder.