Yup, we've all done it. Except for me. :shifty:
Usually, I put my CSS files in their own dedicated folder, cleverly entited "css". So for a particular project, I might use the following code in my heading:
<link rel="stylesheet" href="css/mystyles.css" type="text/css" media="screen">
<font color='"#B22222"'>[B]href="css/mystyles.css"[/B]</font> bit tells the HTML file where to find the CSS file.