So ...

Yes, you are right ... We are going to use a handy, lightweight (75KB) plugin, Easy Table of Contents.

Here it is:

https://wordpress.org/plugins/easy-table-of-contents/

Upon activation, go to Settings => Table of Contents.

This is the page where, you can manage general settings for the table of contents like selecting position, choosing a label for the container, etc:

There are many options there but they are very straightforward ...

The point: you can enable/disable the auto-insertion of the table of content for each post type.

If is enabled, the plugin will automatically add a table of contents for any article that has more than 4 (by default) headings.

But of course you can change that value according to your needs:

OK. Let's move forward to the next lesson to see the Appearance options ...