panorama photos on iPhone 6 are too narrow
What I have figured out myself by trying:


Hold the iPhone vertical (Portrait mode). 
Start the picture-taking by pressing the software button.
Turn the phone in the direction in which the arrow points. 
Move the phone rather slow but steadily.


Keep the tip of the arrow as close on the line as possible.

You are shown some kind of thumbnail image. That is the panorama view which you got already recorded. If the whole thing works you can see how the panorama view grows.

By the way: 
You can change the direction of the arrow before you start the picture-taking by tipping on the arrow.


Finalise the picture-taking by pressing the software button again.