My1stApple,
If you are using the stock Apple Camera app on your 7+ there isn't anything you need to do. You should find the Portrait mode button just to the right of the photo mode. When the app comes up it normally is in the Photo mode and the Portrait mode button is just to the right of this.
If you don't see this then you are either trying to use another app than the Stock Apple camera app, or you don't have an iPhone 7+.
Good luck!
If you are using the stock Apple Camera app on your 7+ there isn't anything you need to do. You should find the Portrait mode button just to the right of the photo mode. When the app comes up it normally is in the Photo mode and the Portrait mode button is just to the right of this.
If you don't see this then you are either trying to use another app than the Stock Apple camera app, or you don't have an iPhone 7+.
Good luck!