fixed easy table of contents

hi,
I’ve installed a bunch of TOC plugins, and for now it looks like this one works with ENFOLD.
page: http://gc-wp-test.azurewebsites.net/docs/overview/

The only problem is I want it fixed, so it would stay in place when scrolling.
this is from the plugin’s info:

the widget supports being affixed or stuck on the page so it is always visible as you scroll down the page. NOTE: this is an advanced option since every theme is different, you might need support from your theme developer to learn what the correct item selector to use in the settings to enable this feature.

and in the plugin options there’s this:
Widget Affix Selector

To enable the option to affix or pin the Table of Contents widget enter the theme’s sidebar class or id.
Since every theme is different, this can not be determined automatically. If you are unsure how to find the sidebar’s class or id, please ask the theme’s support persons.
Eg: .widget-area or #sidebar

hope you can help me with this!
Thank you.