GMC Envoy Owner's Manual page 31

2. Slide the guide under and past the belt. The elastic
cord must be under the belt. Then, place the guide
over the belt and insert the two edges of the
belt into the slots of the guide.
1-24
3. Your comfort guide may have an adjuster to
lengthen or shorten the cord. If it does, you
can adjust the cord by following these steps.
3.1. Squeeze and hold the ends of the lock (A)
on the elastic cord.
3.2. Pull the loop to shorten the guide or pull the
cord to lengthen the guide.
3.3. Release the lock.