GMC SIERRA CK2 1999 Owner's Manual page 61

3. 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.
4. To tighten the belt, pull up on the shoulder belt 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.
Make sure the buckle end of the belt is pulled out all
the way.
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