Buick 2004 Century Owner's Manual page 54

3. Pick up the latch plate, and run the lap and shoulder
portions of the vehicle's safety belt through or
around the restraint. The child restraint instructions
will show you how.
Tilt the latch plate to adjust the belt if needed.
4. Buckle the belt. Make sure the release button is
positioned so you would be able to unbuckle the
safety belt quickly if you ever had to.
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5. Pull the rest of the shoulder belt all the way out of
the retractor to set the lock.