GMC 2002 Envoy - Manual (Page 65)

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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.