1- Set the seat height so that when your arms are forward over the pad, your elbows are in line with rotational axis of the machine.
2- Grab the handles and curl the weight up toward your shoulders, while keeping your back flat and feet firmly on the ground.
3- Curl the weight down, and be sure to keep the your arms slightly bent, and repeat.
Tips and Advices:
1- The upper arms should remain stationary and on the pad at all times.
2- Be careful not to allow your elbows to straighten out completely, as that might cause injury.
3- E-Z bar can ease elbow strain.
1- Some gyms may have preacher bench instead of machine, in that case you can use barbell or dumbbell to perform the exercise.
2- Another biceps isolating exercise is Dumbbell Concentration Curl