GMC 2004 Sierra - Manual (Page 63)

4. Pull the rest of the shoulder belt all the way out of
the retractor to set the lock.
5. 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-facing child
restraint, you may find it helpful to use your knee to
push down on the child restraint as you tighten
the belt.
6. Push and pull the restraint in different directions to
be sure it is secure.
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