How to install KDE desktop environment in Ubuntu 12.04/11.10/11.04

As we know that now Ubuntu default desktop environment is Unity. But everybody is not liking Unity so they can try other desktop environment like Gnome , KDE , Xfce , Lxde etc. Today I am going to tell you how to install KDE in Ubuntu. KDE (K desktop environment) is another desktop environment.It is known for its Plasma Desktop, a desktop environment provided as the default working environment