It's a little hard to tell from the picture, but the dress above started as a strapless gown with a straight across neckline.
1) Measure to find the center of the dress and then measure down 2 to 3 inches.
2) Start at the bottom pin and using a heavy duty thread, take a running stitch up to the top of the dress.
4) Tie a knot and then make a chain with the remaining thread and tie another knot
5) Here's what it looks like all finished:
There are several other ways to get the same look, but I find this method is quick and effective.