Well, this is more fitting in the GUI Customization forum, but..
If you want to just apply the icon to a specific file (whether an app. or a folder), just ctrl-click on it, go to Get Info, and drag and drop the desired icon on to the little preview icon in the Get Info window. To restore it, just highlight the preview icon and press delete.
I also recommend
CandyBar for safely replacing system icons, and easily changing almost all system icons quickly.