On the bottom of the P16 mainsail bolt-rope (tack) is a large grommet, tie 1/4" low stretch line to the mainsail at this grommet using a bow-line knot. On either side of the mast at the goose-neck assembly are 2 cheek-blocks with a jamb-cleat with fairlead beneath one of the cheek-blocks. Once the mainsail has been hoisted and secured, feed down-haul line through cheek-block on 1 side, back up through grommet, down through cheek-block on opposite side, back up through grommet and down through jamb-cleat giving you 3:1 purchase. Crank on needed down-haul and secure line using jamb-cleat (v-cleat)
bottom right pic shows you the setup, click to enlarge