The following page contains a link to a beginning Byzantine Chant textbook that explains the system: http://www.stanthonysmonastery.org/music/LearnByz.htm

The pdf is in Greek, but has been annotated with English notes throughout. It also contains embedded mp3 resources so you can hear what the notation is showing.

More details can be found: http://www.byzantinechant.org/notation.html

Including video tutorials: http://www.byzantinechant.org/tutorials.html