How do I properly install the car seat using a vehicle seat belt?

To install with a vehicle seat belt, route it through the correct belt path and buckle it in. For a locking retractor type (Switchable or ALR), slowly pull out all slack; for ELR types, use a locking clip to secure it. Check tightness by pulling on base from side-to-side and front-to-back without moving more than 1” (2.5 cm).