Sit on floor with feet or heels against pulley machine. Grasp low pulley cable bar with shoulder width underhand grip. Lie back on floor with arms to sides.
With elbows to side, pull bar until elbows are flexed. Lower until arms are fully extended. Repeat.
When elbows are fully flexed, they can travel upward, slightly allowing forearms to be no more than horizontal in this lying posture. This additional movement allows for relative release of tension in muscles between repetitions. Also see mechanical analysis of arm curl and question regarding elbow position.