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 set up 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. In the future you won't need to load Qt 4 first when using KDE 4, because by requiring KDE 4 everything needed is loaded automatically.
If you got the same results so far you seem to have a working environment already.
There is always the option to install the bindings from source code, but it is recommended to use the existing binary packages.
To install Ruby you should know how to use your package manager.
Install the package "ruby-kde4" or something similar. Be aware though, that there might exist a package named "ruby-kde", which probably contains the old version of the bindings for KDE 3. Consider furthermore, that you will need a lot of KDE dependencies, if you are working on a GNOME desktop environment.