Adding buttons answer

Hello

I am an old sewer and we used to do midi's LOL I know you don't know
this!!!!! anyway !!!!!!!!!!!!

you make a piece of material either black or something matching what the
colour is in the blouse maybe a bit lacy but not see through cotton
would do
Make a V Like a square folded in half on an angle corner to corner
(like a kerchief)

ok now hem the triangle all the way around and put snaps on the right side
of the triangle and put snaps on each of the 3 corners and on the inside
of the blouse and just snap into place

You will have to be careful to sew on the blouse only through the inside
layer of fabric or the threads will show from the outside

LOL when I was young it was improper to wear low V necks and this is how we
did it

Sometimes the old ways are better

Hope this helps

An Old Sewer