GMC 2003 Yukon Owner's Manual page 68

If your child restraint does not have the LATCH system,
you'll be using the lap-shoulder belt to secure the
child restraint in this position. Be sure to follow the
instructions that came with the child restraint. Secure
the child in the child restraint when and as the
instructions say.
1. Put the restraint on the seat.
2. 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.
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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.