Chevrolet 2009 Suburban Owner's Manual page 82

4. For passenger seating positions with a lap-shoulder
belt and a free-falling latch plate, pull the rest of the
shoulder belt all the way out of the retractor to
set the lock. When installing a child restraint using
a lap-shoulder belt and a cinching latch plate,
skip Step 4 and proceed to Step 5.
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5. To tighten the belt, push down on the child restraint,
pull the shoulder portion of the belt to tighten the
lap portion of the belt and feed the shoulder
belt back into the retractor. When installing a
forward-facing child restraint, it may be helpful to
use your knee to push down on the child restraint as
you tighten the belt.