Installing the Premailer gem
Premailer is awesome. It converts linked and embedded CSS styles to inline style attributes. Alongside this, it converts relative paths, checks links, creates a plain text email and even checks email client capabilities.
You can use the web app and either paste your code or pull the email from a URL. This is pretty useful, however it can get a bit laborious when going through the testing motions. A more integrated approach would be sweet, which is where the premailer Ruby gem comes in.
Once installed you can run commands like
premailer inputfile.html > outputfile.html to generate the inlined email ( full details on command line usage can be found here ).
Super simple, however the install was not (at least on Mac OS 10.9).
The gem requires a HTML parser - Nokogiri which was difficult to install, even through homebrew. However, I found a ruby script that installed Nokogiri successfully.
Simply download the script, navigate to the containing folder in Terminal and run