After trying on the dress and marking for desired neck adjustment, take the garment off and turn it inside out.
Separate the front and back of the dress. Fold the front in half, taking care to match the shoulder seams.
Starting at the fold, draw a curve for neckline in chalk.
Do not alter the shoulder line, use a ruler if necessary to maintain the edge so it will still match the back.
Carefully cut through both layers of the dress front with scissors or a rotary cutter.
Try dress on again to check neckline.