This page should describe how to set up different MUAs to send and receive secure email. The following pages assume you've got the common tasks like key generation and publishing done properly already:
The Elisp package to add security to Gnus is called PGG, there's an info file on it in the Emacs manual.
You'll need to put
<code> (require 'pgg) </code>
in your ~/.emacs. I also had to customize the following variables (using M-x customize-variable):
pgg-gpg-program should be set to the path to gpg ("/opt/local/bin/gpg" for me)
pgg-default-user-id should be set to the private key to use. This is similar to the default-key option on ~/.gnupg/gpg.conf, but is the email address instead of the key id (e.g. " email@example.com" for me).
I had to set pgg-cache-passphrase to nil to work around a bug in the Aquamacs 0.9.7b distribution of GNU Emacs
gnus-posting-styles to associate the holly.skynet.ie "newsgroup" with the proper From and Return-Path headers. I was worried about this because I've only got the key associated with my Skynet email address for now, so wanted replies to go here. Then you need to compose new emails from the holly.skynet.ie group window. This is only needed if you use Gnus to read multiple mailboxes.
That should be it. Now to encrypt or sign email, just select the option from the "Attachment" menu (I agree this isn't very intuitive).