GMC Sierra Denali Owner's Manual page 61

5. Pull the rest of the shoulder belt all the way out of
the retractor to set the lock.
6.
7.
To tighten the belt, feed the shoulder belt back into
the retractor while you push down on the child
restraint. If you're using a forward-facing child
restraint, you may find it helpful to use your knee to
push down on the child restraint as you tighten the
belt. You should not be able to pull more of the belt
out of the retractor once the lock has been set.
Push and pull the child restraint in different
directions to be sure it is secure.
1-54