The Importance Of Shoulder Belts - Ford 2008 F-150 Owner's Manual

If the booster seat slides on the vehicle seat, placing a rubberized mesh
sold as shelf or carpet liner under the booster seat may improve this
condition.

The importance of shoulder belts

Using a booster without a shoulder belt increases the risk of a child's
head hitting a hard surface in a collision. For this reason, you should
never use a booster seat with a lap belt only. It is best to use a booster
seat with lap/shoulder belts in the back seat- the safest place for children
to ride.
Move a child to a different seating location if the shoulder belt
does not stay positioned on the shoulder during use.
Follow all instructions provided by the manufacturer of the
booster seat.
Never put the shoulder belt under a child's arm or behind the
back because it eliminates the protection for the upper part of
the body and may increase the risk of injury or death in a collision.
Never use pillows, books, or towels to boost a child. They can
slide around and increase the likelihood of injury or death in a
collision.
2008 F-150
Owners Guide (post-2002-fmt)
USA
Seating and Safety Restraints
195