Grasp the bar using a shoulder-width supinated (underhand) grip.
Stand approximately one foot away from the pulley and lean backward a little. Your elbows should be by your sides, your arms should be almost fully extended, and the cable should be pulled taut.
Keeping your body still, exhale as you curl the bar towards your shoulders until your elbows are fully flexed.
Once your elbows are fully flexed, allow them to move forward a little (shoulder flexion), just until your forearms are vertical.
Hold for a count of two and squeeze your biceps.
Inhale as you lower the bar to the starting position, with your elbows by your sides and arms almost fully extended.
Comments and tips
Keep your body still. Only your arms should move.
A wavy bar may be easier on your wrists than a straight bar. You can also use a v-bar.
With the barbell and dumbbell curls, the biceps brachii does not experience much tension at both the top and bottom of the repetition because the angle of gravitational pull is directly downwards (i.e. the force vector is vertical). With the cable curl, if you stand approximately one foot away from the pulley and lean backwards a little, as described above, tension will be delivered throughout most, if not all, of the range of motion.