How do I convert this car seat to booster mode?

To convert to booster mode, first loosen the harness straps by pushing the release button and pulling to extend completely, then unfasten the chest clip and buckle. Store the buckle through the seat pad opening. Lift the headrest pad and push the shoulder straps behind it. Pull the harness straps, chest clip, and buckle tongues through the forward-facing belt guides, snap the chest clip halves together behind the seat, replace the headrest pad, and adjust the seat to upright position. (Pages 56-57)