These dresses had a small slit up the side, so I stitched that closed..
Then stitched the bottom edges of the dress together (right sides together) to form the main part of the bag:
Next, with the bag still inside out, put your hand in and flatten out the corner to make a point like this:
And put two pins in a line around 4 cm in from the point, then stitch along this line
Now cut the two top parts of the dress to use for the straps, like this:
Before joining them together, turn the raw cut edges of both the bag and the pieces for the straps once and stitch.
Then pin the pieces for the straps to the side seams of the bag - right sides together. And stitch in place. Then turn the top rim of the bag over once more and stitch - this should hide any remaining raw edges
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).
Then fold the ends together so the raw edges are hidden, and stitch.
These dresses had a handy little tube of ribbon on the straps that I then stitched over the seam to hide it.
For a little extra decoration, I took the elasticated part from the back of the bodice of the dress, and an offcut from the strap..
....rolled the elasticated part up and tied it..., then handstitched to the side of the bag!
And here's the finished bag: