Instructions
Download the "Source Multiplayer Dedicated Server" by clicking "View" and then "Tools" in Steam. Double-click the "Source Multiplayer Dedicated Server" and select "Install."
Start the dedicated server by clicking "Source Multiplayer Dedicated Server" by double-clicking on the name once it finishes downloading. Select "Team Fortress 2" from the "Game" drop-down menu, enter a server name and click "Start Server." Click the "X" to close the server. This will create the proper folders in the Steam directory.
Open the Steam install folder, which defaults to /Program Files/Steam/ in Windows, and select "Steam Apps," then "Your Steam Name" and then "Source 2007 Dedicated Server." Select "TF."
Place your custom maps files into in the "Maps" folder.
Find custom maps that you have played before by going to "Steam Apps," then "Your Steam Name," then "Team Fortress 2," then "TF" then "Maps." Any custom map you have played in "Team Fortress 2" will be saved here as a .BSP file. Copy those files to the "map" folder for the dedicated server.
Start the dedicated server and select the "Console" tab. Type "map" and then the map's name to start a custom map.