I started out with this:
I made these cuts:
The sleeves I set aside for other projects. The collar actually had a few tears and stains (I don't want to know where they came from, vampire?) so I discarded that portion. I turned under a portion of the top (front and back) to make a casing for the elastic pieces I used:
Then I added the elastic and stitched it in place at both ends:
I raided my scrap fabrics for some white eyelet and created two cute straps and a small rosette.
It was that easy! I did take in both sides a teeny bit under the arm so that it would fit a bit better but here is the finished product:
