WordPress Child Themes for the Working Human

Create a new directory in wp-content/themes — let's call it danktheme in this example. Create two files in that directory: style.css and functions.php with the following contents:
/*

<?php

That's it: now you can select "Your Themename" in the admin interface. If you don't want to use twentytwentyone as the parent theme — because it's not the most recent one when you read this, for example — replace it in the style.css. Yes, in the stylesheet, I'm not making this up, rules are rules.