2000 Buick Century Owner's Manual - Page 59

D If your vehicle has the child restraint locking feature on the shoulder belt retractor, pull the rest of the shoulder belt all the way out of the retractor to set the lock. 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. 1-47