In comparison to the installation process on Microsoft Windows or Apple Mac OS you have to deal with the fact, that there are a lot of different Linux-based operating systems. So there are also different ways to setup your environment for kdebindings.
Test Bindings Installation
Maybe the Qt/KDE bindings are already installed. For a quick check, enter irb in a shell to start the interactive ruby shell. Try to load the bindings.
You can load bindings for KDE 4 (korundum4) or only Qt 4 by a require. Later you won't need to load Qt 4 first when using KDE 4, because on requiring KDE 4 everything needed is loaded automatically.
If you got it so far you seem to have a working environment.
There is always the possibility to install the bindings from source code, but it is recommended to use the existing binary packages.
Installation On Linux-based Systems
After installing Ruby you should know how to use your package manager.
Install the package "ruby-kde4" or something like that. Be aware, that there maybe exist a package "ruby-kde", which probably contain the old version of the bindings with KDE 3. Consider furthermore, that you will need a lot of KDE dependencies, if you are working on a GNOME desktop environment.
Installation On Mac OS
Installation On Windows