How to insert a button into a webpage and execute some functions through Chrome extension?

I am new to Chrome extension developing and currently I am trying to build a Chrome extension that allows users to mark selected online contents and save them. Now I get stuck at the "Inserting a Button" part.

What I wanna do is as soon as users select any text on a webpage, a button pops up right above the selected text like how it works on Medium. And I've found this repository Medium Editor that shares the kind of buttons. But I am not a sophisticated so even though it is very detailed but it's too detailed to me and I don't even know how to use it.

All I want right now is a popup button appears on a mouseup event thats uses "onclick" method to communicate with the script and execute the target functions.

How should I do it?

Super thanks in advance!