How do I transition from harness mode to booster mode?

To install the backless booster, remove the seat back by loosening the harness straps and pushing down on the harness release lever. Remove harness straps from splitter plate, lift the red backrest locks with the car seat in the 6th recline position, slide them together, and remove the seat back from the base by pushing it up and lifting it out. (Page 7)