GMC Sierra 2005 Owner's Manual page 64

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 belt to tighten the lap belt portion
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 out of the retractor once the lock has been set.
7. Push and pull the child restraint in different
directions to be sure it is secure.