I chopped the sleeves and the neck off the men's shirt to make a trapezoid shape and cut off the lower half of the v-neck.
I took in the sides of the blue shirt to the width of the v-neck and also to make it a more flattering, less boxy shape. Then I turned the blue shirt inside out and tucked the v-neck inside, with right sides together and raw edges lined up. I pinned the shirts together all the way around, then stitched the seam with a stretch stitch.
I folded the seam allowance toward the black shirt and top-stitched it down. Here is the finished dress - a close-up and a full-length view.
It was very easy, and it took less than an hour. And it is so comfy! Thanks for reading!