Missing instructions:
Use the left stick to navigate the menu, and the left shoulder button/bumper—not a face button—to select.
While playing, red squares will appear and grow. Position your crosshairs over them with each analog stick, and press or hold the shoulder buttons to shrink them down to nothing. If you don’t, the square will turn darker, which will hurt you but also make it worth more points, until it turns black and stops growing.