GMC 2003 C4C042 Owner's Manual page 69

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, feed the shoulder belt back into the
retractor while you push down on the child restraint. If
-
you're using a forward
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.
facing child restraint, you may