GMC 2008 Yukon Denali Owner's Manual page 78

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.