Dampen the shirt with lukewarm water
Lay the shirt flat on a clean towel
Gently pull the fabric in the areas you want to stretch
Hold the stretched shape with heavy objects or pins
Let the shirt air-dry while stretched
Wear the damp shirt and move around to help it loosen
Soak the shirt in a mix of water and hair conditioner for 15 to 30 minutes
Rinse lightly, then stretch the shirt by hand
Repeat the process gradually if more stretch is needed
Avoid using high heat, which can shrink the shirt again