How do I properly install and use the Graco Slimfit 3-in-1 in rear-facing mode?

For rear-facing harness mode, ensure the car seat is installed at the correct recline angle using the level indicator on the car seat, and position the harness straps at or below your child's shoulders. The weight range for this mode is 5-40 lb. Always ensure proper installation before use, as incorrect installation can lead to serious injury.