Lay the shirt flat on a clean surface
Cut off the sleeves along the armhole seams
Decide how low you want the armholes and neckline to be
Mark the cut lines with chalk or fabric marker
Cut the neckline if you want a deeper tank top shape
Trim the sides if you want a more fitted look
Cut slowly and evenly to keep both sides symmetrical
Try the shirt on and adjust the cuts if needed
Stretch the cut edges slightly if you want a rolled look
Wash the shirt to soften the edges after cutting