How do I secure the car seat in my vehicle with a lap or lap/shoulder belt?

Place the car seat firmly against the back of a forward-facing vehicle seat, thread the seat belt through properly, and buckle the vehicle belt. Push down hard with your knee in the middle of the car seat while pulling and tightening the vehicle belt. Perform a safety check by twisting and pulling the car seat front to back and left to right to ensure it is very tight and the vehicle belt stays tight. (Pages 22-23)