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!