High square necklines are easy to do for a vintage look.
I'll show you how.
New Look 6000 has a wide round neckline. I've extended the shoulder seam in by 1.5cm and pinned the square on an angle so it covers my bra straps.
I've used full lining in the animal print for this dress. What you see above is the interfaced neckline and the lining now stitched to the dress. The square needs to be clipped before trimming back the seam allowance.
I've clipped the corner and stopped before the stitching. Then trimmed back the seam allowance.
Here's the neckline post trimming and ironing.
To get it looking sharp, I've pinned it ready to topstitch.
Those pins make top stitching easy.
Now the zipper in sewn in and the lining and side seams are ready to finish.
The back neckline is square.
The front neckline is now square and more vintage like.