Adjust the cable to be as low as possible, then grab the straight or EZ curl bar with an underhand grip, spaced at roughly shoulder width.
Standing straight, curl the bar upwards to approximately shoulder height, then slowly extend your elbows until they are almost fully straightened. Keep your core activated to maintain a straight back throughout. Ensure that your wrists stay aligned with your forearms. Repeat for the desired amount of repetitions.
Avoid leaning back to pull the weights or using momentum on the way down.