How would I set up a server on a separate router?

I’m trying to set up a server on a second router. I’m doing this because I want to use a different router for my server and my primary router for my personal devices. I need to forward a port so that other devices connecting to that port can only connect to my second router. How would I go about connecting these two routers?

Question by Vinny on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.