I recommend creating a directory named style-guide in your site's root directory. I think it would be awesome if I could go to anysite.com/style-guide/ and check out that site's style guide.
You should be able to go to yoursite.com/style-guide/ and see how your live site's CSS affects base elements.
Below the custom styles for the boilerplate, you will add in your own custom stylesheet(s) which you use on your live site.
To create custom patterns like buttons, breadcrumbs, alert messages, etc., create a new .html file and add your HTML markup into the file.
Save the file as pattern-name.html into the markup/patterns directory inside of your style-guide directory.