Making a device accessible with SSH from another device over the internet

I'd like to run a Raspberry Pi, on which I'd like to log in with ssh, connected to my router. Then I would like to access it from another RPi, which is connected to another router with physical (and network) distance. I assume I need - a static IP for both RPis and the router. - ssh on both RPis. - a somehow stable internet connection and router. Did I forget something? How to set this up? How to give my RPi a static IP address?