Over sized no more! I'm sure there's tutorials all over the web for this but here's what I did.
Here's one of the shirts in question:
I am super fancy. This t-shirt is super awesome because I friend of mine picked it up for me at Comic-Con one year (and scored me some original art) and I love it it pieces. As you can see, it's not exactly a great fit. I fixed that.
The first step is to gather up this shirt and another that actually fits. I chose a shirt that fit but was a little loose because if I chose something more form fitting I'd cut off some of the design of the blue shirt. If you have a design on your shirt, check measurements for that before you do anything drastic.
You will also need a marking pen or chalk and fabric scissors.
Fold both shirts in half and place the smaller shirt on top of the large shirt and tuck in the sleeves. Then trace around the smaller shirt, leaving a distance for a seam allowance.
Cut along your line. Next, trace out your new sleeve size by placing the new cut out on top the sleeves. Trace and cut. These sleeves didn't need much resizing, but every shirt is different.
Turn each piece, body and two sleeves, inside out with right sides facing each other. Sew along the body sides (do not sew the armholes!) and the sleeve bottoms according to your earlier seam allowance. I'll note that the sleeves, while not traced with a seam allowance they should be sewn the same as the body of the shirt. The picture is blurry, but hopefully it shows what I mean.
Time to attach sleeves. Place the sleeves inside the armholes, right sides together with seams matching. Sew together.
I do like me some bright colors. To put in perspective how long I've held onto some of these shirts that pink one is a band t-shirt for a garage band from high school. Junior year. Yeah, that's right. Seven-ish years I've held onto it because I love pink and I knew I'd wear it one day.
The Godfather one is awesome because it is extra long and would probably look pretty cool with some skinny jeans or leggings.