To start, in Finder, click Go > Utilities, then double-click Terminal.
This opens a command shell.
At the command prompt, usually "$", enter:
ssh -l username ssh.afn.org
where username is your AFN account username
The first time you log on, you'll get a warning message similar to:
The authenticity of host 'freenet1.afn.org (188.8.131.52)' can't be established.
RSA key fingerprint is 42:90:da:2b:8b:1b:21:43:d3:8b:3e:be:78:72:c8:f4.
Are you sure you want to continue connecting (yes/no)?
Answer "yes", typing out the word in full.
When you've done that, you get another warning that can be ignored:
Warning: Permanently added 'freenet1.afn.org,184.108.40.206' (RSA) to the list of known hosts.
Then you'll be prompted for your password.
Enter your AFN password, and you'll be logged on after a short period for credential check. After a brief message flash, you should then see the options menu.
Future log on sessions will usually skip the authentication warnings, though if the SSH service has been moved to another freenet machine, it will happen again for the new machine. Enjoy!