GMC Sierra 2008 Owner's Manual page 498

6. Stand the wheel blocks on their shortest ends, with
the backs facing each other.
5-106
7. Place the bottom edge
of the jack (A) on the
wheel blocks (B),
separating them so that
the jack is balanced
securely.
8. Attach the jack handle, extension, and wheel
wrench to the jack and place it (with the wheel
blocks) under the vehicle toward the front of the
rear bumper.