Fukuoka to Kurokawa Onsen

Dear Sir, Which is the most convenient method to travel from Fukuoka to Kurokawa Onsen. IF it is by bus,if I would like to travel from Hakata Bus Centre to Kurokawa Onsen, can I purchase the bus ticket via website online ? can I purchase bus ticket in advance , which is now ? As we are planning to travel from 26Dec to 28Dec2015. In view of December period is festive season, we would like to ensure that the bus ticket is available for us.
Appreciate your help to provide guidance to me. Sorry that I am unable to understand Japanese language, please kindly provide me English version. Thank You for your great help.

You simply cannot get there by train - you can take a bus, or drive. Firstly, did you see the page on Kurokawa at
http://www.japan-guide.com/e/e4575.html
It has a lot of useful info.
About reserving a bus seat, the bus is Sun-Q, and there is actually a website you can use at
http://www.atbus-de.com.e.jo.hp.transer.com/
but to me it looks difficult to use. You can also reserve a seat over the phone or go to their counter (I doubt anyone can speak English very well though). The Japanese page allowed you to reserve a seat up to a month in advance. The translated page only worked for a few days in advance though.
Are you going to use a Sun-Q pass for your travel? If you are going to other places besides Kurokawa within 3 days, it can save you some money.
Fukuoka has a number of good places to see as well. Look through
http://www.thejapanfaq.com/fukpic.html
https://www.youtube.com/playlist?list=PL7LcPcmoSg-j4RLa6iC7ayvBAbOn8-0...
to see the best of what you might be interested in.