Go to Layout. At the bottom you will see the menu Line Numbers. Click.
Then a new window called Line Numbers will open. Click Add line numbering and from Numbering choose any of your preferred options. For a document of several pages for throughout line numbering, choose Continuous.
That's it.
Thanks go to my colleagues Dr. Angelika Schöner and Dr. Herwig Marbler for the tip.
![Razz :P](./images/smilies/icon_razz.gif)