How to add an external CSS file in master page

Hi,

We're running a SharePoint 2013 (Office 365) team site. I'd like to use the same formatting in all my site pages. I currently use a script editor web part to apply the formatting changes which is cumbersome. How can I utilize a css file so that all of the site pages use the same formatting?

The site that I want to apply the formatting to is a sub site of the main site. Only the main site has a "Styles Library" if this matters.

Thanks.

Brian