You need to remove enough of the lining at the top so it will meet the neck facing. Place the facing pattern piece around the top of the back linings and add the pattern seam allowance. Note the CB seam is 1 inch wider.
Now with RST sew the centre back seam with the patterns seam allowance.
Next we add the box pleats. They are marked 1 inch further in from the seam line you just finished. The first pleat is marked from the top down approximately 4 inches then angled out towards the centre back seam edge.
The second pleat is marked in a similar manner. This pleat should finish around the top of the hip and be roughly centred on the waistline.
Finally the bottom pleat is an upside down version of the top pleat.
Now at this step you can loose any darts. My darts run to the bottom edge of the back so measure the width of one dart (at the bottom) and taper your last pleat (adding the dart width). If your jacket has no darts or fisheye darts, that don’t run to the bottom edge, you don’t need to adjust the pleat at the bottom!
This picture might help!