This is the hard part, setting up a port on your router can get difficult however if you want to play with your friends this is a necessary unless you use something like Radmin or a free/paid server to host your Rimworld server.
The easiest way to find out how to port forward is to google "how to port forward on (your internet provider)"
We cannot provide a tutorial on this as every internet provider and router are different.
To check if your port is open, use: https://ismyportopen.com/
IMPORTANT: make sure the port you have opened has been listed in the "Config.json" and that the server is running or it will show as port closed, if the port is still showing port close, check that you have allowed to open ports with your internet provider
Finding your local IP[]
Press the windows key + r
Type "cmd"
in the box that will pop up and click "OK"
In the command window, type "ipconfig"
It should say ipv4 that’s your local IP
Allowing under your firewall[]
This is a very important step, if you do not do this step it may cause disconnects and stop people from joining the server
Open the windows security page and click "allow an app through the firewall"
Then find "Open World Server" and allow both public and private and click OK