Stand with your feet shoulder width apart to give your body a solid base. Grip the bar with your palms facing in spaced about shoulder width apart and bend slightly forward. Pick the bar up off the ground using your legs (not your back) and extend your arms, giving your biceps a full stretch. Curl the weight towards your body, pause briefly at the top, and then return the weight back to the starting position.
It is important to allow your body to move naturally throughout this exercise. Your back should not be completely straight and rigid, but instead should move with a slight sway as you curl the weight up. This does not mean that you should cheat and be sloppy with your form, it just means that you should move in a natural motion to prevent injury and allow you to handle more weight. It is also important that you do not curl the weight so high that the tension leaves your biceps. Simply curl the weight to the point where you feel a full contraction in your biceps, and then let the weight down.