Install a OTR-capable IM program:
- Pidgin for desktop/laptops
- Pidgin needs a separate OTR plugin
- Adium is a good alternative for OS X that does not require a separate plugin.
- Android and iPhone users can install Gibberbot
Once this is complete, configure your client to connect to Facebook chat.
Now all you need to do is have your friends do the same thing. If you chat with someone via Pidgin clicking on the "Not private" indicator in the lower-right side of the chat window will switch to secure mode.
Your friend will be able to read the messages, but anyone else will just see something like this:
If you try to start secure communication with someone who does not use OTR, they will see this:
email@example.com/ has requested an Off-the-Record private conversation <http://otr.cypherpunks.ca/>. However, you do not have a plugin to support that.
See http://otr.cypherpunks.ca/ for more information.
From then on your conversations will be private between you and the recipient. You can talk with some assurance that nobody else is reading over your shoulder.
Spread the word - If everybody reading this convinces six of their friends to set up OTR we'll all be having private communications in a very short time.