GMC 2006 Envoy Owner's Manual page 26

4. Push the latch plate into the buckle until it clicks.
Pull up on the latch plate to make sure it is secure.
If the belt is not long enough, see Safety Belt
Extender on page 1-31.
Make sure the release button on the buckle is
positioned so you would be able to unbuckle the
safety belt quickly if you ever had to.
5. To make the lap part tight, pull up on the
shoulder belt.
1-20
The lap part of the belt should be worn low and snug on
the hips, just touching the thighs. In a crash, this
applies force to the strong pelvic bones. And you would
be less likely to slide under the lap belt. If you slid
under it, the belt would apply force at your abdomen.
This could cause serious or even fatal injuries. The
shoulder belt should go over the shoulder and across
the chest. These parts of the body are best able to take
belt restraining forces.
The safety belt locks if there is a sudden stop or crash,
or if you pull the belt very quickly out of the retractor.