Ford 2003 Explorer Owner's Manual page 89

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.
SAFETY SEATS FOR CHILDREN
Child and infant or child safety seats
Use a safety seat that is recommended for the size and weight of the
child. Carefully follow all of the manufacturer's instructions with the
safety seat you put in your vehicle. If you do not install and use the
safety seat properly, the child may be injured in a sudden stop or
collision.
Seating and Safety Restraints
89