GMC Sierra 2000 Owner's Manual page 267

For an underbody spare tire carrier, store the tire under
the rear of the vehicle in the spare tire carrier. To store
the tire:
1. Put the tire on the ground at the rear of the vehicle
with the valve stem pointed down and to the rear.
2. Tilt the retainer downward and through the wheel
opening. Make sure that the retainer is fully seated
across the underside of the wheel.
3. Attach the ratchet, with the UP mark facing you,
near the hook at the end of the jack handle. Insert the
other end, on an angle, through the hole in the rear
bumper and into the hoist shaft.
4. Raise the tire fully against the underside of the
vehicle. Continue turning the ratchet until the tire
is secure and the cable is tight. The spare tire hoist
cannot be overtightened.
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5. Make sure the tire is stored securely. Push, pull, and
then try to rotate or turn the tire. If the tire moves,
use the ratchet to tighten the cable.
Return the jack, ratchet, wheel wrench and jack
extensions to their location behind the passenger's seat.
Secure the items and replace the jack cover, if there
is one.