Using a child theme is a very smart thing. This way all your extra CSS is saved within the child theme, meaning if the main theme's updated your child theme won't be changed but stays the same. Besides, it's a lot cleaner and better organized. The easiest way's probably to use a plugin for it - some kind of child theme generator.

• install plugin
• follow the steps of the plugin to create a new child theme
• BEFORE activating: go through your Customizer-settings (of your parent theme) and save what you want to paste into the child theme's Customizer (or style.css), such as colors, fonts, header settings
• AND: make a copy of your parent theme's custom CSS (if you used that before)
• now you can activate your new child theme
• adjust the settings of your child theme accordingly
• add the child theme's folder (in wp-content > themes ) to your WP backup
• a child theme usually comes with a function.php and a style.css
• now you can open the child theme's stylesheet in your editor of choice and add your custom styles there
• each time you make changes upload your child theme's style.css onto the server (overwrite the style.css in the sometheme-child folder)

… here you go