Introduction: T-Shirt Mod: Boxy to Foxy
First, iron your shirts and turn them inside-out. Line up the template at the shoulders and trace around it with tailor's chalk.
Pin along the new side seams, and cut off the sleeves at the new armhole. Run the new side seams through the serger.
Lay out the old sleeve under the template t-shirt and mark the new underarm and armhole.
Serge the underarm, then cut the shoulder curve.
Flip the sleeve right side out and set it inside the armhole of the shirt. Pin both sleeves into their armholes.
Serge the sleeve seams and you're all done!