Ok, this one is easy. You actually run the same binary like the game - but with an additional option editor.
For instance if you want to create desktop icon for editor and you already have icon for game, you should do the following (Windows XP):
- Drag your game icon with right mouse button and when you drop it on the desktop choose Copy Here from the menu.
- Click right mouse button on your new icon and select Properties. Properties window will appear with Shortcut tab selected.
- Go to Target inputbox and add space and word editor at the end of the line. E.g. if you had "D:\Games\Unreal Tournament 3\Binaries\UT3.exe" in your Target inputbox you would have "D:\games\Unreal Tournament 3\Binaries\UT3.exe" editor there now.
- Optionally you can switch to General tab and change shortcut description on the desktop right here - otherwise you can do it later.
Now double click the icon and be patient - it might take a while till you see UnrealEd splash-screen.