I started with a shirt that was too big for me.
I cut it off right under the arms.
Then I used the fabric from the back of the shirt top to make straps.
Then I used elastic thread in the bobbin to put in a few lines of shirring on the back.
That helps it to stay a little more tightly against me. That way I don't flash anyone if I bend over.
I'm pleased with the way it turned out. It fits well and is comfortable to wear. I made sure to make the straps wide enough to cover my bra straps so that wouldn't be an issue either.