Once the kayak is in position on the roof rack and the straps are laying over the kayak it is time to strap it down. First, ensure the straps are laying flat against the kayak and are not crossed. Then bring slide each kayak strap so that the buckle is laying against the hull of the kayak. Bring the other end under the crossbar and back up to meet the buckle. Thread the kayak strap up through the clamp by pushing the button on the clamp thereby opening a slot for the strap to fit into. Pull the straps to take up the slack but don't pull too tightly at this point. Do the same to the other strap.
Now that the kayak straps are threaded through their buckles, its time to tighten them up. Pull each strap down, allowing the straps to slide through the buckle. These buckles are really one-way clamps that allow the straps to slide through them one way (against some resistance) but not the other. To undo a strap simply push the button and give loosen it up. You want the straps tight. It is ok if a plastic canoe or kayak seems to compress in the process as they will regain their form once they are free.