Sit on curl machine. Grasp lever handles with underhand grip. Align elbows at same pivot point as fulcrum of lever while placing back of arms up on pads.
Pull lever handles toward shoulders. Return handles until arms are fully extended. Repeat.
Back of upper arms should remain on pad throughout movement. If machine has secondary lever (as shown), it is not as essential to position elbows in-line with the primary fulcrum, as would otherwise be required.
Long head (lateral head) of biceps brachii is activated significantly more than short head (medial head) of biceps brachii since short head enters into active insufficiency as it continues to contract. Long head of Triceps Brachii is only significantly activated as antagonist stabilizer when it approaches passive insufficiency as elbow nears full flexion. Also see movement on preacher machine with arms positioned lower.
Antagonist Stabilizers (see comments)