You have to use a link element to link the CSS to the head of your html document.
An example would be "<link href="file.css" rel="stylesheet" type="text/css" />"
Then you open up that html document and see how it looks. Another way is on Internet Explorer. Go to Tools -> Internet Options -> Accessibility (near the bottom) -> click the tick box "Format documents using my stylesheet" -> browse and upload your CSS.
Then, just go web-browsing, and everything should be formatted according to your CSS