Currently work-in-progress, incomplete.
EDIT: turns out there's already something similar out there.
As you might already know, port forwarding is required if you want to host multiplayer games. For RVGL, the only port you need to forward is 2310 UDP.
Note that the method is always somewhat different, depending on the router, and some ISPs (Internet Service Providers) don't even allow port forwarding, therefore it's always better to watch a video or follow a guide that uses the same router as yours. (Portforward.com, Youtube)
For that, you'll need to know your router's brand name and model number. The former one shouldn't be hard to find, the model number is usually written on the back of the router or in the manual. Some manuals can also be found online, just make sure you search with your router's model number.
So, I'm gonna assume you didn't find any help online.
First, open up the command prompt. The easiest way to do that is to go to the start menu and search for "cmd".
When you click on it, it should bring up a black window.
Now, type "ipconfig" and press Enter.
That should give you a bunch of information, but you will only need two things: what's next to IPv4 and Default Gateway.
IPv4: this is your local or private IP. The IP assigned to the device you're using. For more information: https://www.iplocation.net/public-vs-private-ip-address
Default Gateway: this is your "router's IP" and you will need this to access your router settings.
Note: If there are two IPs, a longer and a shorter one, ignore the longer one.
Open any browser, copy the address next to Default Gateway (192.168.0.1 for example) and paste it into your browser's address bar, then press Enter.
What you should see now is your router's web-based setup page. Here are some examples:
It will almost always ask you to log in using a username and a password. You should check the manual or go on the internet again to find the default username and password for your router.
The username, by default, is often admin, or user, and the password is often admin, or password. Try combining these and see if you can log in, but only if you couldn't find the password and the username anywhere.
Once you're in, try looking for a menu like this:
It's often named "Port Forwarding/Mapping", "Gaming", "Applications" or "Services". Port filtering is not the same as port forwarding. This, however, can look very different at other routers, so here's how you can recognize it:
It will almost always ask you for three things at least: the port or the port range, the protocol, and your local IP (some routers will ask you to specify the device instead of setting the local IP).
For the port, write 2310. It may ask you for a starting and an ending port, in that case, since you're only forwarding one port, write 2310 for both. Do the same for "external" starting and ending ports, if you see something like that.
For the protocol, choose UDP, "Both", or UDP/TCP.
For the IP, go back to the cmd window, copy the address next to "IPv4" and paste it in. Sometimes you might only have to input the last few digits. If it asks for an internal and external IP address, only do this with the former one and write 0.0.0.0 for the latter.
If you're lucky enough, it won't ask you for anything else, and you can do all the forwarding at one place.
Make sure RVGL is added as an exception at Windows Firewall.
People can join your lobby using your public IP. The easiest way to get it is to visit a website like https://www.whatismypublicip.com/. Remember: Your public IP is constantly changing.