Step 1: Forward the ports that steam uses on your router

Your network must be configured to allow Steam access to the following ports (in order from highest to lowest priority for QoS users):
Steam Client

UDP 27000 to 27015 inclusive (Game client traffic)
UDP 27015 to 27030 inclusive (Typically Matchmaking and HLTV)
TCP 27014 to 27050 inclusive (Steam downloads)
UDP 4380

Dedicated or Listen Servers

TCP 27015 (SRCDS Rcon port)

Steamworks P2P Networking and Steam Voice Chat

UDP 3478 (Outbound)
UDP 4379 (Outbound)
UDP 4380 (Outbound)

https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711

Step 2: Allow Steam through your firewall (if it isn't already)

Step 3: Start Team Fortress 2

Step 4: Start a server
Note - For MvM games you must set player count to 32. Also you will want to set a password.

Step 5: Once the game has started you need to open up the console and type the following.

To enable to console goto - Options -> Keyboard -> Advanced and check the box labelled 'Enable Developers Console'

The tilde key (~) will bring up the console.

ip [your external ip address]

http://www.whatsmyip.org/ - If you want to find out your external ip address

sv_lan 0
sv_pure 1
heartbeat

Players should now be able to join the map. If not run the 'map [map_name]' command in the console and reload the map.

For MvM to start a vote to increase the difficulty type 'call vote'

That should be all you need to get a server up and running.

Let me know if you run into any problems.

SoS