When my four daughters were younger, I did the following alteration many times. We would get some "hand-me-downs" from a friend and there would be some perfectly good T-shirts but they were too large. The knit fabric was good quality so I would make the t-shirts smaller for my girls.
First, I would determine if the neck opening was small enough for their size, not stretched out, otherwise it would be too big and immodest. I didn't want to have to resew the neckline if I could avoid it.
Then I would fold the shirt exactly in half, trying to keep the grainline straight and smooth. Notice the picture below. The green shirt is an XXL that my teenage son HAD to have, so I told him that I could "downsize" it for him. The Nike shirt on top is the size that he typically wears right now. Sometimes I use a pattern for the body and the sleeves, but sometimes I just use the shirt that fits my son as the pattern.
You can see where I cut new sleeves and cut off the bottom hem and sides. I try to retain the sleeve hem if possible.
Find the center of the sleeve top and match it up to the shoulder seam. The armhole length should be the same as the sleeve cap seamline. The stretch knit will ease in just fine. Pin the raw edges together. Sew or serge the seams.
After sewing the flat sleeves, pin the side seams and sew up into the sleeve seams.
Since the original shirt had a black coverstitch on the outside of the side seams, I decided to duplicate this since I have a coverstitch serger. The end result looks really good and made my son happy.
I finished the bottom hem by using a double needle and sewing a double topstitched hem. Look at my blog on sewing a T-shirt hem for further instructions.
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