GMC 2003 C4C042 Owner's Manual page 21

3. Pick up the latch plate and pull the belt across you.
Don't let it get twisted. The shoulder belt may lock if
you pull the belt across you very quickly. If this
happens, let the belt go back slightly to unlock it.
Then pull the belt across you more slowly.
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4. Push the latch plate into the buckle until it clicks.
Pull up on the latch plate to make sure it is secure.
If the belt isn't long enough, see "Safety Belt
Extender" at the end of this section.
Make sure the release button on the buckle is positioned
so you would be able to unbuckle the safety belt quickly
if you ever had to.