Check back at this article for the previous post: http://wp.me/p8f30-hc
So, you have a back placket or zipper opening.
You must add boning to the edge of the back opening. (If you have added a placket, skip the next step).
Make an opening the top seam (near the fold) on each side large enough to insert boning.
Insert boning the length of the opening plus one inch on each side of the opening.
Top stitch alongside the boning with a zipper foot to hold it against the folded edge.
Stitch under the boning to prevent it from slipping out at the bottom.