How do I make a sidebar index?

Site URL: https://www.jodistewartart.com/portfolio-drawing-watercolour

I've attached categories to all my artworks so the viewer can search through different collections and available works but how do I stop ALL the works showing up when you initially arrive at the page? I want it to be like this site https://www.jamesguppy.com/galleries.php

but I can't figure out how to put an index in.

TIA - this is breaking me

The page url doesn't exist. Can you check it again?