Here is the clear answer (in baby steps) as to how to get this to display the "Nest Selector Inside of"...as it is unclear in the movie tutorial.
After you give a class name to your Rich Text Block container on the page:
You select an individual element within the rich text block, for example, the h2 tag... you then see the Add Selector & blank field in the right hand panel.
Yes, you still click the little plus icon in the blue square to add a new class. Now, the name in the purple box will appear! Click it to select it.
It is only now that the "Nest Selector Inside" becomes available. Click it, and now you may use the panels below to style out the element as needed.
To apply this to a dynamically generated web page, open that page (meaning, not in the CMS editor, but on the screen so you are viewing it as a proper web page). - Select the Rich Text Block on the page, and now click the pencil (edit) icon in the panel at the right... sift through the list of class names to find the one you had assigned that Rich Text Block to begin with. Once you do that, the styles you intentionally set up will apply to the layout.