This page aims to be a brief step by step procedure enabling digital revolutionarys to quickly and easily protect their emails from mass survielence grabs and targetted spying. For more details see the EFF's lengthy document.
It's really that simple once things are configured. Below is an annotated image of the Thunderbird screen.
First, you should download Thunderbird and GnuPG from the links above. May as well have them ready and sitting in your downloads folder.
GnuPG does nothing on it's own. We need it to enable the proper installation of Enigmail. Just go through the installer. There's no need to generate a key since Enigmail can do that for you.
Thunderbird in an email client. Most people are accustomed to browser-based email clients (like ymail.com). Thunderbird is an email client that runs on your local machine instead of in the web browser.
Go through the installer. Leaving all options as default will be just fine. You don't need to understand the options, you can't really mess anything up.
After Thunderbird is installed and you open it for the first time, a dialog window will show up on the screen asking you whether to create a new account or add an existing email account. Choose add an account, and fill in your email information on the next screen. Thunderbird will automatically take care of all the complex email configurations for you.
Once you have an email account setup in Thunderbird, you'll need to install the Enigmail plugin. You can actually install this plugin from within Thunderbird. From the Thunderbird menu (hit alt if you can't see it), click Tools → Addons. Now use the search bar to search for “Enigmail” and install the plugin.
But how do you send an encrypted mail? ...Well, the way this encryption works is you generate a 'key pair', a private key, and a public key. Everyone just shares the public key, it can be mailed directly to the firstname.lastname@example.org and no harm done. When a correspondent wants to send a message just to you, they use that public key to encrypt the message making it unreadable to everyone, even themselves! That's a pretty powerful concept, if you find it hard to believe, just google it, put on your math hat, and become fully educated. So let's go on about sharing keys public keys, shall we?
So, when you send an email, right before it gets sent to your mail server, it turns into a big block of encrypted text looking like this:
To test that your email client is in fact encrypting your messages, login to your web mail the way you usually would, and open the 'sent' folder. Click an email you sent and thought was encrypted, it should look like the text in the above image, completely encrypted. Your recipient will also know that it's encrypted when he or she logs into their webmail and opens your message, it should be unreadable in webmail. But now you can read these types of messages in the Thunderbird email client.