GMC Sierra Denali 2006 Owner's Manual page 54

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. You should not
be able to pull more of the belt from the retractor
once the lock has been set.
7. Push and pull the child restraint in different
directions to be sure it is secure.
8. If the airbag is off, the off indicator in the inside
rearview mirror will be lit and stay lit when the
key is turned to RUN or START.