Step 1: Cut out a long denim strip that fits right above your boob. It should wrap just around the top of your boob.
Step 2: Pin fabric to the strip, pleating it slightly as you go. I used about 1.5 yards of silk. Sew that shit down.
Step 4: Pop in a zipper. Use this tutorial to make an non invisible zipper look like an invisible one: http://www.costumedesignblog.com/?p=37
Step 5: Since most people look like shower curtains when they wear maxis without some sort of waist, then we're going to add a pseudo waist. What you basically want to do is pin together folds of fabric and regular intervals. This will create a fake waist without being too tight. It will still be super loose and flowy. Sorry for the shitty picture.