Chevrolet 2005 Silverado 1500 Pickup Owner's Manual page 32

If the belt is not long enough, see Safety Belt
Extender on page 1-29.
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.
3. To make the lap part tight, pull down on the buckle
end of the belt as you pull up on the shoulder part.
1-26
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.