During the Regency period when necklines were at a fashionable low, women, for modesty, warmth, and variation, often wore chemisettes to fill in their necklines.
Image courtesy of Sense and Sensibility Patterns
You can make a chemisette from scratch, but here's another option that incorporates a perhaps beloved blouse you may not wish to part with. This is was a linen blend blouse I bought several summers ago. The collar being my favorite thing, I decided to salvage it by creating a chemisette that I could wear to fill in the scoop necklines of Regency gowns. It could also work with v-neck sweaters.
Try on the blouse and decide how long your chemisette will need to be to cover the bust and leave enough room for a casing around the ribcage. After removing the blouse, you will cut off the blouse a few inches below the bust and at the sides to remove the sleeves and sides.
Narrowly hem the chemisette at the sides. Then turn up the lower edges 3/4". Turn under raw edge and stitch down. You will need a piece of twill tape or 1/4" ribbon long enough to go around your ribcage, plus a few inches for the placket overlap. This you will run through the bottom hem. Try on the chemisette for size Then back stitch in the front to secure the tape or ribbon. Sew a hook and bar where the placket overlaps. Change the buttons if you like and take pride in your resourcefulness!