How to use multiple stylesheets

I’m currently creating a new theme based on _S.

Could somebody explain to me how I can split up my style.css file into several to make them easier to manage? For instance, I’d like to have a layout.css, typography.css and other.css file stored within a CSS folder:

-css
–layout.css
–typography.css
–other.ss

How do I set this up. Presumably I need to add something to the header.php file?