This is a free pattern from Sew Beautiful Magazine- Issue #93. Each issue has a fold out in the center with free patterns for items featured in the magazine.
You can see in the pictures below how the backs overlap and tie in a bow. The pattern was fairly easy to put together. It does require a buttonhole under the arm on one side to thread the sash through.
I have made 10 - 15 of these sundresses in all types of combinations from this pattern. Further down are some pictures of two of the ones I made. Their are a couple of different styles that come with the dress. One of them has a border along the hem of a coordinating fabric. And for those who don't smock, their is a style that has a plain bodice instead of a smocked insert. I wish I had pictures of some of the others that are more colorful but you can see from the magazine how cute it looks with the bright fabrics.
This is the other version without the smocking and a collar.