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.
Step 4: Tip: dealing with too long or too tight sleeves
The first picture shows what the sleeves will look like - they roll a bit, but they're much more comfortable. :D
I use a clear ruler and a rotary cutter to cut the same amount off each sleeve. Just flatten the sleeve well and use the cuff edge as a guide for cutting - that way you'll still have a nice slant to the sleeve and the edges will stay nice and straight.
Step 5: If you're worried about the neckline unraveling
I've got several of these, varying in age from brand new to 6-8 years old. I've never had one unravel, but here's what to do if you're worried.
Hand stitch next to the neckline of the shirt, over where the front and back pieces of fabric meet. That will tack it together for you. :D