My fabric has a very soft hand. Instead of underlining, I interfaced all of the pieces with a nylon fusible.
In order to stabilize the jacket and prevent stretching across the shoulders and armholes, I drafted a back stay or "half-back interfacing". I used cotton muslin (or you can use any thin fabric), but DO NOT USE FUSIBLE INTERFACING for this part! This is drafted by cutting it 6" to 8" inches from the neckline and about 3" below the armscye.
It's basted to the neckline, armhole and the side.
For the shoulder seams, I eased the back and used stay tape.
Note: I pinked the edges of the back stay that was hanging free to prevent fraying.
