Customized Dash (Launcher / Taskbar) Icon in Ubuntu

To customize application icon in Ubuntu:

1. Create a .desktop file in ~/.local/share/applications/
   e.g. /home/user/share/applications/<application name>.desktop
   
2. Add the following content to the desktop file

[Desktop Entry]
Encoding=UTF-8
Version=1.0
Type=Application
Name=ApplicationName
GenericName=ApplicationName
Icon=/home/user/application-icon.png
Exec=/home/user/application/application-executable
Categories=Category;Subcategory

Done!