All subsequent steps must be performed as the "root" user on the server. SSH into the server as "root". You should have either the password or the passphrase from section II to do this
As of this writing the latest stable version of SquirrelMail is 1.4.5. Download the source tarball squirrelmail-1.4.5.tar.gz (it can be found at http://sourceforge.net) to the temporary directory /var/tmp.
# cd /var/tmp
# ls -l squirrelmail-1.4.5.tar.gz
-rw-r--r-- 1 root root 591226 2005\u201312\u201308 11:53 squirrelmail-1.4.5.tar.gz