How do I transition the Graco Slimfit 3-in-1 to booster mode and what are the requirements?

To use the highback booster mode (for children 40-100 lb), remove the car seat harness and use your vehicle's seat belt instead. The vehicle's lap belt should fit low across the child's hips, and the shoulder belt should rest across the center of the child's shoulder and chest. Ensure proper seat belt positioning for safety.