Documentation
Get twitter2ii
You can get twitter2ii by checking it out sung SVN from svn://kore-nordmann.de/twitter2ii/trunk.
Installation guide
Install II: http://www.suckless.org/programs/ii.html
Configure in config.php
Configure the startII script and start it in background, for example using bash:
./startII & disown
Set up the crontab to run the post.php and update.php scripts from time to time, for example:
*/1 * * * * cd /home/twitter2irc/src && php post.php */5 * * * * cd /home/twitter2irc/src && php update.php