Look for a pattern for an A-line dress with a jewel neck. You can always make the skirt and sleeves longer if this look isn't an option on the envelope. You might even want to look at some nightgown patterns for some ideas.
The dress is probably a pullover -- no zipper, but it might have a small opening at the neck in back with a button, snap or hook & eye fastener. This would allow the neckline to remain modest, while giving you room to pull it over your head.
Apply a separate casing to the inside at your natural waistline. Don't create the channel for the drawstring by folding the dress on itself and running one seam; take your bias tape and attach it by sewing it along both edges so it lies flat against the dress. Remember to leave an opening in the casing at the center front of the dress.
Probably the most important thing is choosing a woven fabric with lots of drape. Most cottons are a little too stiff to get the flowing quality this dress has. A really nice rayon would probably work well; look for a fabric that has that sort of hand.
Take a look at pattern #408B on this page for a dress that's similar in style: http://www.commonsensepatterns.com/csp_main.htm
or this Butterick pattern #B50149: http://www.butterick.com/item/B5049.htm??tab=list/dresses&page=all
And this McCall pattern#M5583: http://www.mccallpattern.com/item/M5583.htm??tab=list/dresses&page=all