Lincoln NAVIGATOR 2015 Owner's Manual page 32

Child Safety 31
Children and booster seats vary 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 following drawings
compare the ideal fit (center) to a shoulder belt uncomfortably close to
the neck and a shoulder belt that could slip off the shoulder. The
drawings also show how the lap belt should be low and snug across the
child's hips.
If the booster seat slides on the vehicle seat upon which it is being used,
placing a rubberized mesh sold as shelf or carpet liner under the booster
seat may improve this condition. Do not introduce any item thicker than
this under the booster seat. Check with the booster seat manufacturer's
instructions.