Create a portal 2 mod

The old tutorial was obsoleted by DLC2. This is the new method, as described by Lpfreaky90 and Felix Griffin.

Step 1

Create a folder for your mod in Steam/steamapps/sourcemods and rename it to the name of your mod. Open that folder and create two subfolders: "cfg" and "maps".

Step 2

Open the "maps" folder and create a folder called "soundcache" inside it. Copy the file called "_master.cache" from Steam/steamapps/common/portal 2/portal2_dlc2/maps/soundcache to this new soundcache folder.

Step 3

Create a gameinfo.txt file in your mod's root directory. Use this template: "GameInfo"

   {
       game        "Your Mod's Name"
       title       "Your Mod's Name"
       gamelogo    1
       SupportsDX8     0
       SupportsXbox360 1
       FileSystem
       {
           SteamAppId 620      // Portal 2 has ID 620
           ToolsAppId 211      // Hammer
           SearchPaths
           {
               Game                |gameinfo_path|.
               Game                portal2_dlc2
               Game                portal2_dlc1
               Game                portal2
           }
       }
   }

Step 4

Use a program like GCFScape to open Steam/steamapps/common/portal 2/portal2_dlc2/pak01_dir.vpk and copy the file called resource/ui/basemodui/mainmenu_new.res to (your mod)/resource/ui/basemodui. Edit it, then pack it into a new VPK using portal 2/bin/vpk.exe or GCFScape.

Step 5

Restart Steam. If it all worked, your mod should be listed. Congratulations!