There are lots of existing resources out there for automatically inlining CSS after writing your template, so we recommend you use whichever one you like best.

Zurb Foundation has one with a web interface: http://foundation.zurb.com/emails/inliner.html

I personally prefer Jon Kemp's open source inliner, which is a good choice if you want to do that step locally: https://github.com/jonkemp/inline-css