(sorry, the picture is a little blurry and there too much light)
Last year our 2nd daughter was on the JV soccer team at her high school. When we ordered t-shirts we accidently ordered a child size shirt and sweatshirt for her so they were a little small. Well, they actually fit her little brothers (ages 9 & 7). That's pretty bad. I couldn't believe I had done that. So, to try to make it up to her I told her I would take in my t-shirt to fit her. She tried to talk me out of it, but I told her it was more important for her to have one than for me to.
I used one of her other t-shirts to measure for the size, and took it in to fit her! She was so excited! I hope you can see in the above picture how I cut down the large shirt. I cut the shoulder seam out first, to raise the sleeve higher on the shoulder, and then cut out the side seam all the way up to the end of the sleeve. Last I hemmed the bottom of the shirt.
And we made the shirt into a small, from a large and my daughter has a new t-shirt again.