For example, here's a link to the "main.css" file in the same directory as the document. You can also provide a full url to the .css file to point to style sheets on other web servers. The only part you would change is the "main.css".
<html>
<head>
<title>Document Title Here</title>
<link REL="STYLESHEET"
HREF="main.css"
TYPE="text/css">
</head>
<body>
Document Body Here
</body>
</html>