GMC 2008 Sierra 1500 Pickup Owner's Manual page 29

5. To make the lap part tight, pull up on the
shoulder belt.
It may be necessary to pull stitching on the safety
belt through the latch plate to fully tighten the
lap belt on smaller occupants.
To unlatch the belt, just push the button on the buckle.
Before you close a door, be sure the belt is out of
the way. If you slam the door on it, you can damage
both the belt and your vehicle.
1-25