The short blue prom dress, as seen in the first picture, was way too short. You can tell, just looking at the proportions, that the skirt is going to hit mid-thigh or higher. But fortunately, there is a way to make the dress longer, as shown in the second picture.
This dress had a few things going for it:
First of all, it has pickup points (bunched skirt).
Second, the skirt is a balloon skirt (gathered, then wrapped around and attached to an inner lining).
Both of these design features take up length. Clipping the pickup points and detaching the skirt from the inner lining added about 6″ to the dress .
Making the necessary alterations, then finishing the resulting raw edges, took only about an hour. Here are the steps.