GMC Yukon XL 2015 Owner's Manual page 133

3-64 Seats and Restraints
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.