GMC 2004 Envoy Owner's Manual page 54

5. Pull the rest of the shoulder belt all the way out of 6. To tighten the belt, push down on the child restraint,
the retractor to set the lock. pull the shoulder portion of the belt to tighten the lap
portion of the belt and feed the shoulder belt back
into the retractor. You may find it helpful to use your
knee to push down on the child restraint as you
tighten the belt.
1-48