Basically, it's a large facing in a contrast fabric. Here's the back facing:
Notice how wide it is and how it extends all the way to the sleeve.
Here's the front facing:
(Note: another way to do this is to line your whole bodice in your contrast fabric, but that may get too bulky, depending on the fabric you're using. If you wish to both line and have a contrast facing, you could stitch the facing to the lining.)
To make the pattern, start with your bodice back. Here's a simple bodice back in half scale.
Crease the pattern and mark your fold line.
Make a mark for the top of the zipper at the bottom of the fold line.
Next, draft the facing. It should start 2" below the top of your zipper and extend all the way to the shoulder.
Draw it in with a curved ruler.
Trace the facing area (the area in pink below).
Make a new facing for the bodice front that is 2" wide at center front and extends to the shoulder.
1. Cut the facings out in your contrast fabric and apply a lightweight interfacing.
2. Stitch the facings together at the shoulder seams. Press seams open. Finish the outer edge of the facing.
3. Insert the zipper in your dress back so that the stop ends at your zipper mark.
4. Pin the facing unit to the ourside of the dress, right sides together. Stitch around the neckline, ending at the back, where the zipper starts.
5. Trim and grade seam allowances on the neckline, clip corners, then understitch the facing. Turn the facing unit to the inside and make sure the back corners are turned out neatly.
6. Turn the facing unit to the outside along the fold line. You will need to clip the seam allowance at the zipper stop so the facing can flip out. Press lightly along the fold line.
7. Slipstitch the back facing to the zipper tape.
9. On the inside of the dress, tack the facing in place at the shoulder seam, to the sleeve seam allowance.
Make sense? I hope so. This is the simplest kind of fold back detail you can do. You could also make a shaped fold back, with a scalloped edge for instance. Lots of possibilities! I'll do another post on that when I get back from our little Thanksgiving trip. Speaking of which, Happy Thanksgiving!