It's easy. Go to File, choose Page Setup. Here you have three options - Margins, Paper and Layout.

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.