This technique I learned from Susan Khalje and it’s great when you have to stabilize a V such as a dress back or a sweetheart neckline.
Check out her tutorial on Threads here.
Essentially you sew a piece of silk organza on the right side of the fashion fabric right on the stay stitching line
Now do the all important snip right to the stitching line
You can now turn the facing to the inside and press so that the stay stitching is just to the inside
Catch stitch the facing and the folded over fashion fabric
This treatment definitely needs a lining, so be sure to fell stitch a lining in.
Relax and Enjoy,