Really, it’s as simple as… make a class that implements RawInputListener and then register it with the InputManager.
For way better more advanced input mapping, you can also look at Lemur’s InputMapper which wraps InputManager and provides a bunch of nice stuff like easy scaling, remapping, combo-keys, etc… It can be used without using the rest of Lemur.
…though some other parts of Lemur are useful on their own also: