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.
Step 1: What you'll need:
Step 2: Center the bowl
Center the bowl an inch or two below the existing neckline. Make sure you're leaving an equal amount of space from the bowl edge to each shoulder.
Step 3: Follow the edge of the bowl
Once you've made one good pass, remove the bowl and clean up any spots that may not have been cut all the way through with the rotary cutter.