GMC 2004 Sierra 1500 Pickup Owner's Manual page 69

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-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.
To remove the child restraint, just unbuckle the vehicle's
safety belt and let it go back all the way. The safety
belt will move freely again and be ready to work for an
adult or larger child passenger.
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