Loop kayak at one end.
Step 1: Other Side
Loop other end of kayak.
Step 2: Around the Rack
Wrap it around the roof rack.... Or a safe stationary location.
Image is purely for show. Make sure it can't come off of rack.
Step 3: Slip
Slip the long end threw the other loop.
Step 4: Car Loop
Grab an inexpensive door loop and pick a spot to lock it down. Clip It on and away you go...