First cut the skirt from the bodice of the dress like this:
 
 
Then stitched the bottom edges of the dress together (right sides together) to form the main part of the bag:
 
 
 
 
Before joining them together, turn the raw cut edges of both the bag and the pieces for the straps once and stitch.
 
 
 
Now you need to join the straps together. Cut to a little longer than the desired length for your strap (try it out for size on your shoulder here).