add CSS to theme

I am a newbie to WordPress so thanks for being oversimplified. I downloaded Tinymce Advanced and it says theme doesn’t support customized styles for the editor. It says to create a css file named editor-style.css and upload it to theme’s directory then enable the setting. I’ve tried to find tutorials but none I can follow. Help please…

https://wordpress.org/plugins/css/