1. Lay the top out flat, making sure the bottom edges, underarms, and shoulders are matched up, and over the top, lay a shirt that is of a similar material. Mine are both t-shirt knits, so they have a similar stretch. Match up the underarms of the two shirts.
2. You can see that I've pinned around the edges of the white shirt, creating a line so it will be the width of the red shirt. Again, since my shirts are both knits, they're pretty stretchy, and I don't have to be too exact. Here's a closeup of the pinning.
3. Remove the top shirt, and go stitch down the line of pins. After the first side, it doesn't hurt to try the shirt on quick- being careful of the pins. Make sure it's starting to fit right. If you don't like it, now's the time to use the seam ripper and try again (I did!). Make sure you follow the curve of the underarm. If this seam is too angled, you'll have ugly, uncomfortable puckering. Here's a picture of the underarm of my shirt; I know the seam's a bit difficult to see, thanks to the matching thread!
4. Do the same for both sides. When you've done this, try it on. Is it better? I had a good 2-3 inches of excess on the sides, so I went ahead and trimmed them off.
And here's the after shot again. Okay, it's a little wrinkled right now, but a vast improvement, I'd say, and definitely a much more wearable top.