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.
Move your hands in to a shoulder width or slightly closer to shift demand from the inner portion to the outer portion of the biceps.