Over the last three or four days I have been unsucessfully trying to build KDE with Qt5. It wasn't
untill yesterday morning that I managed to get some instructions which pointed me to building
the correct branches of Qt and KDELibs, and Plasma-Frameworks. However CMake is now giving
this error:

CMake Error at /home/kdedev/local/share/cmake-2.8/Modules/FindKDE4.cmake:98 (messages):
ERROR: cmake/modules/FindKDE4Internal.cmake not found in
Call Stack (most recent call first):
CMakeLists.txt:5 (find_package)

Could someone please write some proper instructions on how to do this please?

Unfortunately concise build instructions are a little hard to write, as the system is still in quite a bit of flux, particularly as the frameworks are still being ported and rearranged into their new structure (which should hopefully make them easier to use and more flexible).

In general, make sure you are building the following:
- Qt5: from 'qt5.git', branch 'dev'.
- kdelibs: from 'kdelibs.git', branch 'frameworks'
- plasma-framework: from 'plasma-framework.git', branch 'master'.

Additionally, make sure no KDE 4 or Qt includes or CMake files are present on your system, as they may interfere in the build. Also ensure that the installation prefix is registered in PATH, LD_LIBRARY_PATH and CMAKE_PREFIX_PATH. Those three should be sufficient for Frameworks based builds.

There is a video (tutorial) for building KDE from source, made by Aaron Seigo and it's available in Youtube.
http://www.youtube.com/watch?v=cqnNVmJocR4

Hope it helps