GMC 2010 Sierra - Manual (Page 113)

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.
Make sure the release button is positioned so you
would be able to unbuckle the safety belt quickly if
necessary.
5. Pull the rest of the shoulder belt all the way out of
the retractor to set the lock.
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