The following text is from another guide but i want to share it with everyone:
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The first step to host a server is to forward port 7777 (unless you want to play over LAN, in which case ignore this paragraph). To do so, check the various links and posts in #faq on the RFMP discord server: https://discord.gg/jdbyAzj.

If you are on Windows and would like to host a dedicated server (one which you yourself cannot join), first find RFMP in your Steam library and click play. Now, instead of choosing to play the game, choose to launch the dedicated server hoster. Follow the instructions on there and your server will be up for everyone to join.

If you are not on Windows or you would like to join your own server, start the game as normal. From there, hold down the multiplayer button. You will see a gray progress bar appear. When it reaches the end, the in-game server hosting menu will pop up. Enter the needed information and click "Create" to open up your server.

NOTE: The in-game server has a few limitations: no special characters in the name or password, the name must be more than three letters, and no spaces are allowed.

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A copy of the Discord faq for ppl who dont want to join the Discord:
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Server guide for dumb dumbs! (Works on non dumbs dumbs):

Hosting:
There are 3 main ways to host a server
1) You can enter the game and create a server with out forwarding port 7777.
These are by far the worst out of the 3 server types the issue with this method is that not all players who try to connect can join as when the port 7777 is not forwarded the connections are done threw NAT (Which is unreliable)

2) You can enter the game and create a server with the port 7777 forwarded
These servers aren't the worst but certainly let all users connect and play. How ever these servers can struggle with larger numbers of player. Over all a good server choice

3) (Windows only) You can forward the port 7777 and launch the server threw the dedicated console
These are the servers all servers aspire to be, They can handle large numbers of players and run very smoothly and consistently (tick rates set to 60). To start one of these boot up the console and type help to get started.

Joining:
Open the server browser, hit refresh and click the match you wish to join.

Please note that starting the game will clear the server from the server list. This is done too avoid ghost servers if you wish to play with your self (>.>) open an instance of the game and then start the server in another instance.

Port forwarding:
All routers are different and I cant really type up a universal guide to that, your best bet are these links (Forward the port 7777). If you do end up port forwarding hmu and I'll set you up with Host!

http://lmgtfy.com/?q=How+to+port+forward

https://portforward.com/

http://www.tomshardware.com/faq/id-3114787/open-firewall-ports-windows.html

https://gtm.steamproxy.vip/app/346120/discussions/0/405690850609661167/

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=398108587


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I would recommend you this vid:

http://www.tomshardware.com/faq/id-3114787/open-firewall-ports-windows.html
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Big love to this Community! <3 Please host a server!