How do I install the Diono Radian 3RXT Safe+ forward-facing?

After placing car seat in the back seat, use LATCH or vehicle seat belt to attach it securely. For children under 40 lbs without a tether point, place SafeStop between harness and seat frame. Attach tether strap for added stability if available. Tighten straps by pulling out any slack until no movement of more than one-inch occurs when grabbing the sides. Adjust adjustable bottom down if child is over 25 lbs. (Page 37-41)