How can I turn a normal dress into a backless dress and it still look good?
What you probably want to do is look at patterns of backless dresses to figure out how they are constructed. I’d be concerned about the back not draping properly. Don’t experiment on your dress. Make another out of muslin if you are going to try to cut your own design. I’m pretty sure a boat neck will have to have some kind of closure at the back neckline, or tape at the shoulders to hold it on your body. Otherwise it could slip down the front, and not in a pretty way. You probably also need plenty of facing and interfacing along the cut out in back so it has some shape and doesn’t look like a hole. This is major reconstruction. I think you should buy more fabric and another pattern if you want a backless dress.
