Documentation is unclear about custom operations

On this page https://doc.qt.io/qtinstallerframework/scripting.html#registering-custom-operations the code to create a custom operation is describbed, but once the CPP file has been written, the documentation doesn't describe what to do with it.
In which folder must it be placed?
Are some commands needed to include it in the installer?
Do we need to recompile something with this file?

An example would also be great.