Lay the T-shirt flat on a clean surface
Smooth out any wrinkles
Cut off the sleeves along the armhole seams
Cut the neckline lower if desired
Trim the shoulder seams to widen the straps if needed
Adjust the armholes for a singlet shape
Cut slowly and evenly on both sides
Check both sides for symmetry
Try it on and make small adjustments
Remove loose threads
Wash the shirt after cutting to soften the edges