Buick 2004 LeSabre Owner's Manual page 52

5. Pull the rest of the lap belt all the way out of the
retractor to set the lock.
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6. To tighten the belt, feed the lap belt back into the
retractor while you push down on the 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.
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.