The Free Software Foundation has a new project aimed at getting people to install and use email encryption software. They have published a guide and infographic that demonstrates in simple terms how (and why) to set up GnuPG and then how to use it. They have versions for Linux, Mac OS X, and Windows.
It’s not very technical but will probably still be too difficult for your Aunt Millie. Of course, that’s why she has you. The real difficulty, as I’ve said before, is to convince people to take the trouble.
There’s an interesting project trying to address that problem. End-To-End is a Chrome extension that adds end-to-end encryption. Since Aunt Millie probably uses something like Gmail, this may be an easy way to get her using encryption. The extension is still being developed and not yet available (although they have released the source code) but it does bear watching. Some commentators have issues with this particular implementation but it does demonstrate what must be done: the day to day use has to be transparent or Aunt Millie simply won’t use it.