Solutions to refashion a strapless dress

Just cut two rectangles long enough to go over the shoulder from front to back.  This will vary depending on the size. Gather the ends and stitch them on the dress by hand or machine.  By hand is the simplest way to go. 

Here we added a piece of tulle over the shoulders to match the dreamy tulle skirt. 

Another look is to make a matching shrug.  I extended the front sides on the pattern as it was very narrow. Most shrug patterns are.  It is easy just to draw another line to make it wider.