A detailed tutorial on how you can create your own MNE Scan plugin directly using the QtCreator can be found here in .pdf format.
Another good starting point is the DummyToolbox plugin, which you can find under \mne-cpp\applications\mne_scan\plugins\dummytoolbox or here. Please note that the DummyToolbox plugin is commented out by default in the plugins.pro file. For example, you could make a copy of the DummyToolbox, rename it and make changes.
\mne-cpp\applications\mne_scan\plugins\dummytoolbox