Chevrolet 2000 Tahoe Owner's Manual page 35

Pull up on the latch plate to make sure it is secure.
If the belt isn't long enough, see "Safety Belt
Extender" at the end of this section.
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 down on the buckle
end of the belt as you pull up on the shoulder belt.
1-22
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'd 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's a sudden stop or a crash.