Cut a V shape in the center through both layers like this. I tried it on an old pair to get an idea of how big to cut the opening. The back of the panties becomes the front of the bra.Fold down the center and cut a little extra from the front. I like to make the neckline a little rounder. Then cut a strip of stretch Lycra or elastic a few inches shorter than the neck opening.
Sew the lycra toghether at the short end like so:
Then fold it in half to make a folded ring like this:
Pin the seam to the back center of the bra. I pin the center of the strip to the center of the neck opening to make sure it is even on both sides. Sew the lycra to the neck opening on the right side of the fabric, stretching the lycra as you go
I used the lycra just because I had some that almost matched and wanted to see how it would work out. I actually like using elastic a little better. If you use elastic, sew it to the wrong side of the fabric (stretching it as you go)
Turn the elastic under and top stitch, always remember to stretch as you stitch! I use a decorative stitch, but a simple zigzag works just fine!