Ford Freestar 2006 Owner's Manual page 168

Seating and Safety Restraints
Children and booster seats vary widely in size and shape. Choose a
booster that keeps the lap belt low and snug across the hips, never up
across the stomach, and lets you adjust the shoulder belt to cross the
chest and rest snugly near the center of the shoulder. The drawings
below compare the ideal fit (center) to a shoulder belt uncomfortably
close to the neck and a shoulder belt that could slip off the shoulder.
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.
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