The base of the seat slides up towards the rear of the seat back to form the booster seat.... according to the Ford Galaxy manual (same as the Sharan) - pull the loop at the bottom front of the seat & push the cushion to the rear until it engages. Then... grasp the loop & pull the booster cushion out of the seatback. To pull out the harness belt, press the locking button of the inertia reel on top of the seatback. remove the cover in the upper area of the seatback (velcro fastener) & run the belt webbings through the upper or lower guiding slots. Fold the cover up & press against it. If the belt webbings are positioned in the upper position, run the belt webbings on top of the cover & press against it. (Height adjusted for child's size) Then put baby into seat, fasten & press the button of the inertia reel until the seat belt is fastened tightly. Pull down the top of the harness until it fits tightly to the body. make sure the safety pads are positioned underneath the buckle.
Hope that makes sense - guess it's easier with the seat infront of you.
Thankyou Mr Ford for the concise instructions!