Read the manual!
A stalk on the wheel controls it. Press button on stalk end to turn on. Green CRUISE should appear on dash. Accelerate to desired speed (>40km/h) and press down on the stalk. Now cruise takes over. In this state, hold stalk up to accelerate; hold down to decelerate. The speed you're at when you release it is the new set speed. Pull it towards you or step on the brake to cancel cruise. When cruise is off this way, tap the stalk up to resume cruising. When you are done, press the button to turn cruise off.