How to safely make a child theme

Attempting larger customizations I decided to make a child theme. But since it seems rather complex I tried automating the process by a plugin, picking Child Theme Configurator.

The process seemed fine and simple, however, only the main page looks OK. Subpages, posts, and categories have some other CSS looking completely different.

I expect some CSS wasn’t loaded properly on pages other than the main page. But I have no idea what and how. I presume the Child Theme Configurator plugin does a normalized way of making a child theme. This apparently doesn’t work well with Activello (although Child Theme Configurator itself doesn’t see any problem).

I have seen threads:

  1. Activello Child Theme,
  2. Problem installing Child-Theme

However, I don’t see this file that is mentioned there and since both threads are over 6 years old I expect they might no longer be up to date.

Does anyone have any advice on how to properly make a child theme?