Please follow below resources and see if that helps.
What is the error you're getting? can you please explain a bit where you're stuck.
I just followed the first one and i could see the plugin.
When I click full screen, i can see custom plugins added.
FYI, you will be able to click on Full screen only when you added some text.
I am trying to enable source edit by overriding
But Source edit button is not visible
Not getting icon
I didn't try this way (/apps/dam/cfm/admin/clientlibs/v2/authoring/contenteditor/editors/StyledTextEditor.js), let me try this as well.
In the mean, the below is working for me, may be it's worth trying 🙂