Your new directory will need to go into your standard WordPress themes directory – usually /wp-content/themes/
You’ll then need at-least a style.css file in that directory, with something like the following at the top of it:
/*
Theme Name: My child theme
Theme URI: https://mywebsite.com
Description: My child theme for TwentyTwelve
Author: Your name
Author URI: https://myotherwebsite.com
Template: twentytwelve
Version: 1.0
*/
Note – the ‘Template’ line is the most important, as this is what connects your child theme to your parent theme. Ensure this is simply the name of the parent theme directory, NOT just the normal name of the theme!
If the style.css file is inside your new directory, and this is uploaded to your WordPress theme directory, you should be able to simply activate it via the admin area under Appearance > Themes
If you need more information about this process, please refer to the official Codex page: https://codex.www.ads-software.com/Child_Themes