When you’re making a plunge back or strapless dress (or other garment that doesn’t allow for a bra), a sew in bra is the way to go. Ching from Ching Makes Things! shares her method for sewing bra cups into a garment lining. Her how-to includes how to get the right placement and then how to stitch them in. Go to her tutorial.
[photo from Ching Makes Things!]