Face low pulley and grasp stirrup cable attachment to one side with underhand grip.
With elbow to side, raise bar until forearms are vertical. Lower until arms are fully extended. Repeat. Continue with opposite arm.
An alternative method is to stand facing with side of exercise arm toward low pulley. When elbow is fully flexed, it can travel forward slightly allowing forearms to be no more than vertical. 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.