Well, its quite easy. Open up Synaptic or your favourite package manager, and search for package kdebase. Mark it to install, and all the necessary packages will be marked as well because of dependecies. This will install kde on your machine. After this logout, and on the login screen search the session button (like on the picture here
). Here you can change which destop to use. After you selected KDE, just type the username and password, and KDE will load. Thats all.