Take the Kyoto city bus 205 from Kyoto station and get off at the eighth bus stop Shiyakusho-mae (京都市役所前). It takes about 18 minutes.
A faster alternative would be to take the Karasuma subway until Karasuma-oike (烏丸御池) and change to the Tozai line. After two stops, get off at Kyoto Shiyakusho-mae.
From Kyoto Shiyakusho-mae, walk a block in northern direction, and after you have passed the Hotel Okura, turn right.
May only open in early November, the entrance fee is 800 Yen.
JP: 〒604-0924 京都府京都市中京区河原町通二条下る一之船入町 東入５３８−１
EN: Kosei-in, 538-1 Ichinofunairicho, Nakagyō-ku, Kyōto, Präfektur Kyōto