How should I position the vehicle seat belt when using the Tranzitions as a booster?

When using booster mode, the lap portion of the vehicle seat belt must go in front of the armrests and lie low across the child's hips and thighs. The shoulder belt must lie across the child's shoulder between the neck and edge of shoulder. Never use a lap-only belt across the front of the child, and do not position the shoulder belt under the child's arm or back. (Page 42-43, 62, 65)