GMC 2016 Sierra Owner's Manual page 107

106 Seats and Restraints
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.
4. Push the latch plate into the
buckle until it clicks.
Position the release button on
the buckle so that the safety
belt could be quickly unbuckled
if necessary.
5. Pull the shoulder belt all the
way out of the retractor to set
the lock. When the retractor
lock is set, the belt can be
tightened but not pulled out of
the retractor.
6. 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.
Try to pull the belt out of the
retractor to make sure the
retractor is locked. If the
retractor is not locked, repeat
Steps 5 and 6.