1. Click on the "+" sign on the main menu for "Create Server"
2. Select a map from the scroll down menu (MvM maps for MvM mode obviously)
3. Under the "Game" tab enter a hostname
3a. FOR MvM MODE 32 PLAYERS IS REQUIRED!!!!!!!!! (IMPORTANT)
3b. Otherwise, set it to what you want.
4. Create a server password for private, leave it blank for public
5. Other options are discretionary
6. Launch your server
7. Once in the game, go into the dev console (~ by default if enabled) and type the following (Followed by hitting enter)
Once you have entered that, type the following in console(yourmapnamehere being a map YOU choose)
7a. Your server will reset and load the map up
8. Congratulations, you now have your very own TF2 server!
Port forward port 27015 to allow hosting if you are having issues
sv_lan 0 lists your game on the server list for all to find and allows players to join
Can't get to the Dev console? Enable it in Options > Keyboard > Advanced...
Here are the three MvM map names
EDIT: DO THIS TO CHANGE YOUR PORT
In the launch options for TF2 use +clientport 270xx +clientport 270xx (Replace the x's with numbers you want to use, do not use 27020. And you have to use 05-32, no higher)
Extra help (Thanks to STD|Flinty for these)
List of TF2 CVAR commandshttp://www.dodbits.com/dods/index.php/tf2/66-team-fortress-cvars-september-2011
Port Forwarding guides http://portforward.com/english/routers/port_forwarding/routerindex.htm