How to add a strip of visual editor on top of comments textarea

Simple-to-follow instructions would be something like:

  1. Install the Comment Form Quicktags
    Plugin

But for some reason, you have specified that you want to do it “WITHOUT PLUGINS”. Further, you have specified “simple to follow instructions”.

I do not believe your requirements are compatible. If you want “easy to follow instructions”, then use a Plugin. If you don’t want to use a Plugin, then you’ll have to figure out how to replicate the (non-trivial, and thus, not really “easy to follow”) implementation in such a Plugin.

Why do you not want to use a Plugin?