GMC 1996 Sierra Owner's Manual page 22

3. Pick up the latch plate and pull the belt across you.
Don't let it get twisted.
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.
4. Push the latch plate into the buckle until it clicks.
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.
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.