There is a application development page on the wiki, but it doesn’t mention what to do with all the files it prompts to create and fill out.
The developer section in the manual is quite a bit outdated and we are considering replacing it with a separate developer documentation that is generated from current source code. Meanwhile, looking at the other apps is a better for current information.
That information is available here: HACKING.md · master · FreedomBox / FreedomBox · GitLab
Adding a file with the python module path of the app in /etc/plinth/modules-enabled/ should do it. After that when Plinth is restarted, it will read this file and load the python module and initialize it. This should be covered as part of the developer section in the manual.
Apart from that, if you have any specific questions about app development, please feel free to post them here.