GMC Sierra 2010 Owner's Manual page 77

Here is how to install a comfort guide to the
shoulder belt:
1. Remove the guide from its storage clip on the
interior body.
2. Place the guide over the belt and insert the
two edges of the belt into the slots of the guide.
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.
2-35