to add to Mark S's reply,
it looks like you used the
<<toc-selective-expandable 'TableOfContents'>>
from the how-to page?
and if you tagged it
$:/tags/SideBar
as the how-to page says to, you don't need to worry about that tiddler any further, it becomes a tab/page in the right sidebar and updates itself. You don't need to worry about any other tagging for it.
The next page you would need to focus on would be your "General" tiddler, it gets tagged
TableOfContents to be your root (first) entry of the table of contents sidebar tab tiddler. Your next tiddlers "Chapter 1 - Introduction" and "Chapter 2" get tagged General and they will appear nested(indented under General in the table of contents. To do the tagging, you add them while in a tiddler's create/edit mode or you can do the "new from here" of the parent tiddler you want them children of as Mark S suggested which automatically tags a tiddler created that way.