Line up seams WRONG SIDES TOGETHER!! Very important and opposite of what you usually do in sewing!
Fold the seam back onto itself with the RIGHT SIDES TOGETHER!!! Which will enclose the raw edges that you just sewed.
Now, at this point, you do not HAVE to do anymore to the seam for the dresses I just made. You have a nice neat seam inside and out. BUT, if you want the little inner piece to lay flat you would open up the seam, press and flatten the inside flap, and zig zag stitch all the way down. This will also result in a zig zag stitch down the outside that is visible. But in this case, I think it adds a nice, unexpected detail to the outside of the dress!