GMC 2008 Envoy Denali Owner's Manual page 56

5. Pull the rest of the shoulder belt all the way out of
the retractor to set the lock.
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6. To tighten the belt, push down on the child restraint,
pull the shoulder portion of the belt to tighten the lap
portion of the belt and feed the shoulder belt back
into the retractor. If you are using a forward-facing
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.
If the airbag is off, the off indicator in the passenger
airbag status indicator will come on and stay on when
the vehicle is started.