I agree with the @Twinkly_Keepers. Start with a pattern.
Simplicity is my favorite pattern company because they have the best selection and the easiest step-by-step directions. I know for sure they have a child-sized Rapunzel pattern, but I'm not sure if they have it for an adult (they do have Snow White and Cinderella, though). I did find something that is somewhat close: http://www.simplicity.com/p-1748-costumes.aspx
You can alter it as much as you need to achieve the right look. Hope this helps!