Here are the original shirts.
As you can see, they didn't fit very well.
In a nutshell, you trace a t-shirt that fits you onto the inside of the big shirt, both the body and the sleeves.
Attach each sleeve to the shoulder of the shirt, right sides together, and sew.
Then unite each side and sew together, starting from the armpit.
If you want, you can re-hem the bottom, but you can also skip it if because t-shirt fabric doesn't fray. So this was before...
...and this is after! Big difference, right?!
For more details on this and other refashioning, sewing and craft tutorials, head on to my blog, Cucicucicoo!