Ok, in fact what I am trying to customise and personalise are mouse buttons. As seen from my previous question here, I use SteerMouse to successfully configure the buttons on my Logitech M545 mouse.
However, there are two buttons, the side buttons, that cannot be detected by SteerMouse. After testing, I find that these two buttons, when pressed, actually correspond to keyboard operations. That is why SteerMouse is not able to configure them two.
Therefore, my question is equivalent to configuring two buttons of an external keyboard. How may I do this?