Start: Grab the barbell with a full, underhand grip that's just wider than shoulder width, and stand holding the bar at arm's length in front of your thighs. Set your feet shoulder-width apart, with your knees slightly bent, your back straight, and your abs pulled in. The correct stance will prevent you from leaning back to help lift the weight.
Finish: Curl the bar up toward your shoulders. When it's 6 inches from your shoulders, pause and squeeze your biceps hard for a second or two. Then slowly return to the starting position.
Bending your wrist back slightly, with the heel of your hand leading the movement, can increase the demand on the biceps by taking the wrist flexors out of the movement.