How do I install the Graco Slimfit car seat rear-facing using the vehicle seat belt instead of LATCH?

Recline the car seat to position 1 or 2, store the LATCH straps and top tether hook, and place the car seat rear-facing in the vehicle back seat. Thread the vehicle seat belt through the rear-facing belt path (marked with a blue label), lock the seat belt by pulling the shoulder belt all the way out and letting it retract, then tighten the seat belt while pressing down firmly on the center of the seat. The car seat should not move more than 1 inch (2.5 cm) when tested. (Pages 17-19)