The introduction to Plasma applet development on the KDE TechBase site describes the contents of CMakeLists.txt file used for compilation and installation of a plasmoid. The installation of corresponding icon(s) is not covered there.
Actually, the process is rather simple
- Create an icon, or an icon set
- Name icon file(s) correctly
- Add an icon reference to the .desktop file
- Put an installation line to CMakeLists.txt
- Run cmake && make && sudo make install