As the gem name suggests this is a guard extension. Make sure you get guard first.

Install the gem:

gem install guard-premailer

Add it to your Gemfile if you're using bundler (you should)

gem 'guard-premailer'

Add a basic guard setup:

guard init premailer

• Source is hosted on Github: guard-premailer
• Report issues/questions/feature requests on the Github Issue tracker for guard-premailer

Pull requests are welcome.