As a first step, create a folder inside the wp-content > themes folder. Call the folder twentyfifteen-child, or whatever you like.

The necessary file for a child theme to work is style.css. A desired file is functions.php. These two files go inside the twentyfifteen-child folder.

Create a child theme and add the below code to the CSS file of your child theme.
Theme Name: Child Theme Name
Template: parenttheme
@import url("../parenttheme/style.css");

Make sure you change <parenttheme> to the actual name of the parent theme and call the parent theme’s CSS file within your child theme’s CSS file. Use normal quotes instead of curly quotes.