Not sure what you’re asking but I’ll give it a try. External CSS usually means CSS outside of your HTML file. If I’m not mistaken, that’s already what’s happening when you use CodePen. You have a pane for HTML, another pane for CSS, and lastly a pane for JS.
if you want to try to build the whole page yourself instead of having the codepen template that does a lot of stuff in background, try repl.it, and there create a html+css+js repl (ignore the js file until you have started learning about that)
you will have to link explicity the css file to your html file there