How would I describe the procedure to measure the average velocity of a body in one-dimensional motion

Newton's first law of motion describes a body moving in a straight line. That's one dimensional motion. If you're driving a car, and you have a speedometer, and you drive in one direction only; you're measuring velocity in one dimension. You can also measure velocity in one direction using a policeman's radar device.

There are plenty of other ways to measure the average velocity. Your description will use the tools that you have available.