The Shirt-Shrinking Tutorial.
Materials:
-- A shirt that you'd like to make smaller.
-- A sewing machine (or really good hand-sewing skills, I suppose).
-- Scissors.
-- Measuring tape (optional).
-- Pins.
-- A pen, marker, or other writing utensil.
To begin, put on the shirt. If you'd like to make it shorter, decide where you want to cut it off (make sure to leave a bit of extra length if you plan to hem it) and make a mark with your writing utensil. Take the shirt off and, making sure you've got it laid out flatly and evenly, cut straight across where you've made your mark. Then put the shirt back on.
Now, it's time to work on taking in the sides. While you're still wearing the shirt, use your fingers to judge how much you'd like to take it in. This time when you take the shirt off, turn it inside out, and once again lay it down flatly and evenly. You can either use your fingers to measure or, if you want to be really precise about it, the measuring tape. Then draw guidelines for yourself so that you'll know where to sew, and draw them not only on the main part of the shirt but the sleeves as well (I can't explain this part very articulately, but I think the picture pretty much explains it all anyway.) Pin along the guidelines so that the fabric won't shift around when you're trying to sew, and after the sewing is all finished, cut off the excess material.
Depending on how much you've taken it in, the shirt should now generally look something like this.
The rest is fairly easy, and all depends on your personal preference as to how long you want the sleeves to be and whether or not you want to hem them and the bottom of the shirt. I would recommend using the measuring tape when you shorten the sleeves to make sure that they don't turn out terribly uneven. And if you'd like to hem, simply tuck under about half an inch of fabric and sew it. The final result on my anti-Santa shirt turned out looking like this.