GMC 2007 Yukon XL Owner's Manual page 58

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.
58
3. Be sure that the belt is not twisted and it lies
flat. The elastic cord must be under the belt
and the guide on top.