Cadillac 2008 DTS Owner's Manual page 56

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.
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5. Pull the rest of the shoulder belt all the way out
of the retractor to set the lock.