TinyMCE advanced problems

Hi everyone. I am trying to figure out how to get TinyMCE advanced working correctly. The plug-in states the following “It seems your theme doesn’t support customised styles for the editor. You can create a CSS file named editor-style.css and upload it to your theme’s directory. After that, enable this setting.” Can anyone walk me through that? I’m sure it’s easy but I’m still learning the ropes and don’t want to mess up my site. https://www.camptonheights.org.