1) You will need to set a bench at about a 45° angle. Lie face-down on the bench with your stomach and torso pressed against the back rest of the bench.
Hold the bar under you using a grip where your palms are facing up. Keep your hands at shoulder width apart.
2) Curl the bar up. Focus on using only your biceps - make sure that the elbow is the only joint moving. Once at the top of the motion, squeeze your biceps.
3) Slowly lower the weight back to your starting position.