you'll need to download a Steam tool which is provided for free but needs to be downloaded separately. (I am currently away from home so this computer does not have Steam installed thus my directions may be off).

Open Steam. At the top menu that says Store, Library, Community, ect. click either Library or Community and a sub-menu will appear immediately underneath and one of the words will be "Tools". click Tools and a long list will appear and amongst the list will be Alien vs Predator Dedicated Server, click and download AvP Dedicated Server.

While the program is downloading you will need to create a custom Default.cfg file to allow the server to know server rules and map rotation. Right click on your Desktop > New > Text Document.

Copy and Paste this into your New Text Document:
################################
// Example default.cfg

servername "Server Name Here" // Server Name
rconpassword Password // Set Server Password
maxplayers 18 // Set the maximum number of players allowed in the server
motd "This is a default message!" // Display a message to players when they connect to your server

// Set your ports, replace xx with any two numbers, ports must be unique for each server you run on a machine. Delete the "270xx" to leave the ports at their default values.
authport 270xx
gameport 270xx
lobbyport 270xx
updateport 270xx

// Jungle = map name, DM = game type, tl = time limit, and sl = score limit.

addmap Jungle DM tl=15 sl=20 // MUST be used to create map rotation before using the "host" command

// Set species ratios: Given in this config, for each 1 Preador there can be 2 Marines, For each 1 Predator there can be 3 aliens and so forth.

predatorratio 1 // Any Value 1-4, relative to other species
marineratio 2 // Any Value 1-4, relative to other species
alienratio 3

// Set auto-balancing and friendly fire. 0 = False, 1 = True

speciesbalancing 1 // Enable or Disable auto-balancing for species
teambalancing 1 // Enable or Disable auto-team balancing
friendlyfire 0 // Enable or Disable Friendly Fire

autostartonallready 1 // Auto-Start matches when all players are ready, 0 = False, 1 = True
autostarttime 120 // Lobby Auto-Start Timer (In Seconds)
host // Start Hosting the Server, server will not show up in any list without this command!

//End configuration
####################################
Save your current process if Steam download is complete > File > Save As > save file name as Default.cfg > location = C:\Program Files\Steam\steamapps\common\aliens vs predator

Now customize your configuration settings. Reopen your default.cfg file you just made. Adjust server name, maybe setup a password or leave it alone for no password, max players (I suggest 12), and the message everyone sees upon logging into your server (motd).

Under the port settings change the 270xx number to anything you like but for this tutorial we will make it an easy range for simplicity. Change authport to 27001, gameport to 27002, lobbyport to 27003, updateport to 27004

Router setup is really important; if done incorrectly your server will not work.
1) Find your computer's ip address. Start Menu > Run > cmd > ipconfig
the default ip address of a computer is 192.168.1.???

2) Open your default web browser > address is 192.168.1.1 > username: admin > password: admin > on the tab menu find Applications and Gaming > Port Forwarding (Port Range) > fill in the range field with 27001 to 27004. If your router does not support a "Range field" you will need to put each port number in individually thus using 4 port field lines. Fill the ip address field with your computer's ip address and click enable > save settings > close window.

Continue customizing your default.cfg by adding a map rotation. Follow the rest of the customization process by reading the instructions already provided in the copy/paste portion of the tutorial. Below are extra details.
____________________________
Available Maps
- Jungle
- Ruins
- Temple
- Pyramid
- Gateway
- Refinery
- Docks (DLC)
- Outpost (DLC)
-------------------------------------------------
Possible Game Modes
- DM (Deathmatch) [requires 2 players to begin]
- PH (Predator Hunt) [requires 4 players to begin] [DLC]
- TDM (Team Deathmatch) [requires 2 players to begin]
- IN (Infestation) [requires 4 players to begin]
- STDM (Species Team Death Match) [requires 3 players, one in each species to begin]
- MSTDM (Mixed Species TDM) [requires 2 players to begin]
- DOM (Domination) [requires ? players to begin] [DLC]
____________________________

Lastly, within the Dedicated Server program files is a pdf listing all the admin commands. I suggest memorizing some of them for quick use if needed such as kick, ban and server message.

*This is my config file*

// Example default.cfg

servername US West California // Server Name
rconpassword Password // Set Server Password
// Password. Type Password in front of these slashes followed by the intended password to set a password to join the server
maxplayers 12 // Set the maximum number of players allowed in the server
motd "Hosting this server from HOME computer. Please keep a honorable combat environment."// Display a message to players when they connect to your server

// Set your ports, replace xx with any two numbers, ports must be unique for each server you run on a machine. Delete the "270xx" to leave the ports at their default values.
authport 27001
gameport 27002
lobbyport 27003
updateport 27004

// Set species ratios: Given in this config, for each 1 Predator there can be 2 Marines, For each 1 Predator there can be 3 aliens and so forth.

predatorratio 1 // Any Value 1-4, relative to other species
marineratio 1 // Any Value 1-4, relative to other species
alienratio 1

// Set auto-balancing and friendly fire. 0 = False, 1 = True

speciesbalancing 0 // Enable or Disable auto-balancing for species
teambalancing 0 // Enable or Disable auto-team balancing
friendlyfire 0 // Enable or Disable Friendly Fire

autostartonallready 1 // Auto-Start matches when all players are ready, 0 = False, 1 = True
autostarttime 30 // Lobby Auto-Start Timer (In Seconds)

// Jungle = map name, DM = game type, tl = time limit, and sl = score limit.

addmap Jungle TDM tl=20 sl=100 // MUST be used to create map rotation before using the "host" command

addmap Ruins TDM tl=20 sl=100

addmap Temple TDM tl=20 sl=100

addmap Gateway TDM tl=20 sl=100

addmap Refinery TDM tl=20 sl=100

host // Start Hosting the Server, server will not show up in any list without this command!

//End configuration