The small garden is located behind the shrine. The shrine was built to commemorate and worship those who have died in war for their country and sacrificed their lives for Japan (the meaning of Yasukuni is "peaceful country"). While this purpose appear benign, Yasukuni Shrine is beset with controversy - there are class A war criminals among the people enshrined here. Furthermore, the visits by several Japanese prime ministers to the shrine have caused concern regarding the separation of church and state.
The garden behind the shrine is surrounded by a pond (Shin-ike or pond of god) with two teahouses. There is a waterfall and the valley leading to the pond represents a view deep in the mountains. The stone bridge near to the entrance is the longest stone bridge in a Tokyo area garden. The garden's location behind the shrine means it is usually uncrowded.
10 minute walk from Kudanshita station (Subway Shinjuku, Tozai or Hanzomon Line)
Do not seek to emulate the old masters. Seek what they sought.