Some years ago, a Zen priest would visit the Gojuryu Karatedo Yoyogi Ryushinkan Dojo every month and instruct the students in the practice of Zen meditation. Although this is no longer a part of the regular curriculum at the Dojo, it is still important for students to work on the mental aspects of training, including the discipline of Zen "sitting." The practice of zazen shares many aspects with Gojuryu training, such as a focus on tensing the abdominal muscles, and deep breathing techniques. In fact, Sanchin kata has been described as a sort of "moving meditation."
A practical and relatively clear manual on Zen training in English is "Zen Training - Methods and Philosophy" by Katsuki Sekida.