Lay the shirt flat on a clean surface
Put it on and mark how wide you want the neckline to be
Remove the shirt and smooth out the fabric
Cut the collar off first if desired
Cut a wider neckline from shoulder to shoulder in a shallow curve
Keep both sides even by folding the shirt in half before cutting
Cut slowly and trim a little at a time
Try the shirt on and widen the neckline if needed
Leave the shoulder seams intact so the shirt stays in place
Stretch the cut edge gently to soften the fabric and help it sit off the shoulder