In your HTML document, paste the following between the opening <head>
and closing </head>
brackets.
<link rel="stylesheet" type="text/css" href="X" />
^ The href="X" denotes the location of your file. There's a special consideration to make when denoting this location:
<link rel="stylesheet" type="text/css" href="css/style.css" />
^ The denotation above suggests that inside of the directory containing your HTML file is a directory called "css" which contains a file called "style.css".
<link rel="stylesheet" type="text/css" href="/css/style.css" />
^ The denotation above (using a slash) suggests that the file style.css is located inside a directory in your root called "css".
<link rel="stylesheet" type="text/css" href="https://howchoo.com/css/style.css" />
^ The denotation above is a valid use of an absolute URL.