Here's the garment in question:
I found it (where else?) at Goodwill, and was immediately drawn to the unique neckline. It's a cross between a scoop and a V neck and it's great.
To take it in, I first flipped it inside-out. Then, I took a shirt that fits me really well and laid it on top.
I marked the sleeves with a pen, because I knew I needed to move the sleeves in. As it originally fit, the shoulder seam was way too wide.
Next, I marked the sides of the T.
I cut the sleeves off at the seam and pinned their new location. I put the pins really far in so that I'd have plenty of room to sew the seam. I did the same with the sides, so that the fabric wouldn't shift around under the sewing machine's foot.
Then, I simply sewed-up all the seams and cut-out the extra fabric. Easy-peasy.
In all, this project took me about an hour, but that was mostly because I'd never done it before. I feel like I could do this project again in half the time.
Now I just have to wait 6 months for fall to wear my new sweater. Wahoo!