This guide is for Windows.

1. Install Qt and Qt Creator.
2. Go to http://dev.kofee.org/QtCreator-Doxygen/ and download the appropriate binary, and follow the directions on that page for the (very easy) installation process. The Qt creator plugins folder is inside Qt's folder, then the version number, then lib, then qtcreator, then plugins, e.g. C:\Qt\2010.05\lib\qtcreator\plugins.
3. Restart Qt Creator if it's running to load the plugin.
4. Go to http://www.stack.nl/~dimitri/doxygen/ to download doxygen. I used the windows installer, and installed with all default options.
5. Go to Qt Creator's Tools → Options dialog, select Doxygen plugin on the left, then browse to the location of the doxygen executables. (default C:\Program Files\doxygen\bin\doxygen.exe and C:\Program Files\doxygen\bin\doxywizard.exe)

It should all be setup now. The basics are to press Ctrl+Shift+F3 over any method/member/class, etc., and the doxygen tags will appear, and some parameters are filled out automatically. If you ever want to build the documentation, go to Tools→Doxygen→Build documentation in Qt Creator, and then the documentation will appear in the project root folder under the “html” and “latex” folders. Open up index.html in the html folder to browse the docs locally.