You can set it as an HTTP header in your server configuration: Link:
; rel=preload; as=style
Or as a tag in HTML like this: Link:
</css/styles.css>; rel=preload; as=style
Or as a <link> tag in HTML like this:
<link rel="preload" href="http://www.devx.com/css/styles.css" as="style">
First option is recommended, because this means that that it will initiate a server push event on most HTTP/2 implementations and that the browser will discover the hint earlier in the response headers, instead of later on in the response body.