You can iron the sleeve without the crease and without using a sleeve board. You spread out the sleeve with the underarm seam uppermost, smooth it with your hands. Then pass the iron over the fabric, taking care to stay well within the outer edges. Using the tip of the iron, smooth the fabric at the underarm seam and at the wristband. Turn the sleeve over and repeat with the outer arm upward. Take your time, especially if the sleeve is very full. With the tip of the iron, smooth the gathers at the shoulder seam and wristband. Spread out the sleeve twice more, to treat the areas not ironed during the first two passes. With practice, sleeves take only a minute or two. BTW, iron tops, shirts in this order: collar, yoke, sleeves, front, back. That way the previously ironed parts don’t get creased again.