Runtime plugin, where to start

Hello,
I want to write a plugin that basically needs to apply its own projection at the end of each frame.
I did never write a plugin at all and if I start by clicking New Plugin from the editor. I just get a sample class with two callback methods: StartupModule and ShutdownModule.
I feel like these are kinda useless to me if I want to manipulate the rendering per frame and I don’t know where to start right now.
What do I need to do that my plugin will be called per frame?
What do I need to do to make a prostprocessing plugin?
Where is some useful documentation on this?
I mean the very lil documentation I found is only describing how to create a plugin nothing more.
How do I make the plugin visible for any user?

I did not find my entry point to that, so I would like to get some help on that.