Do not swing the weight up by arching your back. This is cheating! Your body must remain fixed throughout the movement.
Keep your elbows in at the side and do not allow them to come forward when curling the weight up. Focus on keeping all movement in your forearms.
Instructions
The wide grip barbell curl is a good exercise for training the inner part of the biceps. Grasp a barbell with your hands at a wider than shoulder width, using an underhand grip (palms facing up).
Your feet should be firmly on the floor, about should width apart. If you prefer, you can place one foot back. This is the starting position.
Keeping your body fixed (do not swing!), slowly curl the weight up as far as possible.
Squeeze the biceps, and then slowly lower the weight back to the starting position.