Creating responsive emails can be time consuming. Many email service providers have terrible email editors. With this process you won't have to understand email client compatibility issues, and you won't have to write inline CSS.
This flow assumes that you 1) do not have heavy design needs, and 2) you have an existing email system that accepts HTML/text (and you don't want to migrate your list just to send out your responsive email).
First, choose an email service provider, like Mailchimp, whose web email editor sends responsive emails. Then, use Mailchimp's email editor to create your email and send it to yourself via a “campaign,” not the "test" email button. Mailchimp will send the email to your inbox. Use Gmail’s "view original," and you'll find its source: mangled HTML and text. (Side note: emails are sent as both HTML and text in order to support both sophisticated and unsophisticated email clients.)
How to unmangle Mailchimp's HTML/text
The HTML and text are encoded in a format called "quoted printable." To decode them into something usable, head over to this great tool by Mattias Byrnens, mothereff.in/quoted-printable. Copy the HTML section out of the email, and paste it in "Encoded" textarea. Instantly, the decoded version will appear in the "Decoded" area above it. Do the same for the text version of the email. Take both versions of the email, put them in your text editor, and edit them to your heart's content!
To test your edits (and double-check mailchimp’s output), use PutsMail.com. Paste the HTML in the Body textarea and test out various resolutions.
NOTE: uncheck the checkbox that says, "Make the CSS inline when 'Check your HTML.’" All your CSS is already inline—you don’t want PutsMail to screw up anything by being too smart.
To triple-check that your email is beautiful, use PutsMail to send yourself test emails.
That's it! Enjoy!