CSS for sticky sidebar nav

I'm trying to figure out how to turn the text box at the top of our page become a small sticky sidebar nav (basically a table of contents/index) for our main Barcelona page. I've tried a dozen different CSS solutions I've found online but not one of them works and I don't know what I'm doing wrong. 

The section I'm trying to turn into a sticky sidebar nav box is the "On This Page" block. Any help would be so appreciated. 

An example of what I mean is a (less fancy, less custom) version of something like this: https://www.travelandleisure.com/travel-guide/barcelona

This thread might be helpful:

Hey @Ziggy Thanks for this! I think this thread is for 7.0 and my site is on 7.1? Would it still apply?

Can you share link to your site? We can check easier

Thank you @Ziggy