Portal 2/Creating a 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"
GameData "portal2.fgd"
FileSystem
{
SteamAppId 620 // Portal 2 has ID 620
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!
