Buick 1998 Century Owner's Manual page 36

Rear Seat Outside Passenger Positions Lap-Shoulder Belt
The positions next to the windows have lap-shoulder
belts. Here's how to wear
one
properly.
1. Pick up the latch plate and pull the belt across you.
Don't let it get twisted.
The shoulder belt may lock if you pull the belt across
you very quickly. If this happens, let the belt go back
slightly to unlock it. Then pull the belt across you
more slowly.