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.
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.
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.