Wash and dry the shirt on high heat to shrink it slightly
Soak the shirt in hot water, then dry it on high heat
Use a sewing machine or needle and thread to take in the side seams
Pinch excess fabric at the sides, sleeves, or back before sewing
Cut and re-hem the shirt if you need a more permanent smaller fit
Use fabric tape or temporary hem tape for a quick adjustment
Tuck the shirt into pants or tie it at the waist for a temporary smaller look