2003 GMC Sierra 1500 Pickup Owner's Manual - Page 58

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. 1-51