This trick takes only a couple minutes to do, and I've even included a tip about what to do with your sleeves if they're too long or too tight. :D
Please note that this only works on jersey/knits - and no hemming is needed because the fabric won't fray. (Please see the last step for more info!) It might roll a bit after washing, though! You can also do this trick with a piece of chalk, some pins and some nice scissors, too. The rotary cutter is just easier.