GMC Envoy 2003 Owner's Manual page 56

5. Pull the rest of the shoulder belt all the way out of
the retractor to set the lock.
1-50
6. To tighten the belt, feed the shoulder belt back into
the retractor while you push down on the child
restraint. You may find it helpful to use your knee to
push down on the child restraint as you tighten
the belt.
7. Push and pull the child restraint in different
directions to be sure it is secure.
To remove the child restraint, just unbuckle the vehicle's
safety belt and let it go back all the way. The safety
belt will move freely again and be ready to work for an
adult or larger child passenger.