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.
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. 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.
Installation On Linux-based Systems
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.
Installation On Mac OS
Installation On Windows