For Ubuntu 12.04, Qt Creator 2.4.1, Qt 4.8.0 (64-bit), how does one prepare their system for creating plug-ins for QtCreator?

The best advice I can give you is to find a template plugin for Qt Creator ( which is by the way entirely composed of plugins ) by visiting https://github.com/qt-creator/qt-creator/tree/master/src/plugins

You will see that the plugins are developed using C++, and a plugin interface provided by the core of the Qt Creator application. Following the build instructions for Qt Creator itself should cover most of the common prerequisites to the development of its plugins.

For intstalling plug-ins for QtCreator You have to decompression archive of plug-in to folder /home/username/Qt4.8.0/Tools/QtCreator/lib/qtcreator/plugins.