Lay the shirt flat on a clean surface
Decide how short you want the sleeves
Mark the cut line with chalk or a fabric marker
Fold the shirt in half to match both sleeves evenly
Cut one sleeve along the marked line
Use the first sleeve as a guide for the second sleeve
Cut the second sleeve to match
Trim any uneven edges if needed
Try the shirt on and adjust if necessary
Wash the shirt to help soften the cut edges