Openhabian: static network address setting

for the latest openhabian I want to give it a static address.
I know, there are other ways to overcome this but DHCP addresses are in some areas only for mobile devices and not for servers. And, at least, openhabian is a server.
I tried a few things which have been described here around, but nothing really worked for me.

If there is an old method and a new(er) method, I would go for the new method :wink:

Kindest regards and thank you very much for helping a beginner,

The simples method would be to assign a static ip for the device you are running openhab on in your router.

What did you try so far and can your router assign static ip adresses?