Step1: Attach the rope to an anchor on one side of the load, such as the frame of a cartop carrier. Throw it over the load, then pass it under the frame on the other side.
Step 2: Form a loop in the rope above the frame. Several inches farther down, bend a bight in the rope.
Step 3: Pass the bight through the loop.
Step 4: Put a single twist in the loop this creates, then bring the rope end through it.
Step 5: Pull hard on the rope end to cinch down the load, then secure the knot with two or three half hitches. These can be retightened easily, should moisture or shifting loosen the rope in transport.