I'm trying to understand Drupal 8 theming, since I want some custom layouts for some of my pages. I've tried Display Suite module, and it gets me some of the way, but non of the display suite layouts offers the exact layout I need.

How do I make my own Display Suite layouts? I have made a copy of the layout template files from ds/templates to mythemename/templates/generated, and renamed the file as Display Suite tells me to. When I open that file, I see Header, Left, Right and Footer "regions". But how would I go about adding new regions to that template? I understand how to add block-regions to a website, but that seems to be something different than the "regions" in a display suite template.

And if I edit a display suite template, would I somehow be able to rename it to, lets say, "mytemplate", and make it appear in the drop-down box from where you select what display-suite template to use?