Official website: http://eng.shukubo.net/
Great guide that I used when I was traveling: http://www.japan-guide.com/e/e4900.html
From Japan-Guide: Koyasan is most conveniently accessed by Nankai Railways from Osaka's Namba or Shin-Imamiya Stations. Take the Nankai Koya Line from either station to the Gokurakubashi terminal station. A few limited express trains operate directly to Gokurakubashi (80 minutes, 1650 yen, five trains per day). Otherwise take an express or rapid express train (100 minutes, 870 yen, departs every 20-30 minutes), most of which require a transfer at Hashimoto Station along the way.
At Gokurakubashi, transfer to the cablecar which travels up the mountain to Koyasan. The ride takes about five minutes and costs 390 yen. From the top station, it is a ten minute bus ride into the town center (290 yen to Senjuinbashi bus stop). Note that it is not permitted to walk along the street which connects the cablecar station with the town center.
The hike from the station to the town was not difficult at all. To get to Koya town, people typically take the cable car up but I took the Fudozaka trail instead. The hike is roughly an hour to an hour and a half up the mountain.
From the map near the entrance of the trail, the hike looked pretty straightforward, but it wasn't. At multiple points during the ascent, the path often split. The signs were all in Japanese and if it weren't for the occasional local map on the trail, I probably would have got lost or would have had to just back track and hike down the mountain. Eventually though, I did find my way to town and made my way towards Okunoin Temple.
The temple was alright. At this point in my journey, I was beginning to feel templed/shrined out, so once I got to the temple, and looked around, I immediately headed home. The stroll through the cementary to the Okunoin Temple is really nice however, particularly if you walk during the early morning.
During my trip in Koyasan, I kept running into this Korean guy. Eventually, we introduced one another and started up a conversation during our ride home. His name was Jo and he came from Busan.
Later that night, I went out to dinner with Cathy, You, and Ayame. We went to an all you can eat/all you can drink restaurant for 30000 yen per person. It was delicious and I was in great company. It was a delightful end to Japan, maybe even a sad one.