This was a Men's XL before yesterday. My friend got it for me in the store she works at because the design was cool, but it was too huge to wear.
The first thing that I did was to pin a tank top I liked the size of on top of my big shirt. My favorite tank is actually a spaghetti strap, so I cut around the straps with about a 3/4" allowance.
If the back neckline of your shirt comes up higher than the front, then cut out the seam allowance around this neckline.
Unpin your tank top from the shirt. The front and back of the shirt to be resized should be connected at the tops of the straps.
Now is your chance to cut the front neckline lower if necessary. Unfold the front and back and lay the shirt out as one flat piece.
(Sorry it's lopsided)
Lay the template tank top underneath the shirt to be resized, and use it as a guide to cut the neckline lower, being careful to leave a seam allowance and not cut your tank top.
Carefully repin the front and back of the shirt to be resized together again.
Sew up the sides of the shirt with the stretch stitch on your machine, which looks like this: --^--.
Turn up the bottom of the shirt about 3/4" and hem up. Either do two parallel rows of stitches or a double-needle stitch if you have it on your machine.
Take some of your excess fabric or a contrasting color, and cut a strip about an inch wide and long enough that it could go all the way around your neckline. This will be the binding. Switch to straight stitch on your machine.
Fold one end of the strip over about half an inch, and pin down. Ignore that the bottom edge of my binding is folded over, that's because I was using a shirt hem.
Pin right sides together around the neckline/ insides of straps.
Sew together leaving about a 4/8" seam allowance.
When you're done, it should look like this from the outside:
And this from the inside:
Fold the binding inside the shirt and pin down. Then, "stitch in the ditch" with a thread that matches the shirt, sewing in the space where the two pieces meet.
Trim the extra binding material.
Repeat for armholes.