Start by standing in front of a cable machine with a straight bar attachment.
Grasp the bar with a neutral grip (palms facing each other) and keep your elbows close to your sides.
Slowly curl the bar up towards your shoulders, keeping your wrists straight.
Pause at the top of the movement and squeeze your biceps.
Slowly lower the bar back down to the starting position.
Repeat for desired number of reps.
To increase difficulty, try using heavier weights or performing the exercise with one arm at a time.