Beppu to Mount Aso to Kumamoto

Hi
I will be travelling with family to Kyushu in Dec 2016.- 13 days

In brief, my itinerary is as such:

1. Arrive Fukuoka (Hakata) - stay 1 night

2. Next morning make my way to Yufuin (Explore Yufuin in the afternoon)

3. Proceed to Beppu and check-in hotel. - stay 3 nights

4. Safari
5. Jigoku Hells

6. Kokonoe suspension bridge

7. Check out of Beppu on 4th day and proceed to Mount Aso (Explore Mount Aso)

8. Proceed to Kumamoto and check -in hotel - stay 4 nights

9. Kagoshima (daytrip)
10. Nagasaki (via OceanArrow ferry)

11. Miyazaki - Takachiho Gorge

12. Check out of Kumamoto proceed to Hakata - stay 5 nights.

13. Yanagawa
14. Nagasaki - Huis Ten Bosch
15. Some other side trips

My question is for s/n4 & 5:

• I will not be using JR pass at this leg of the journey. Upon checkout of Beppu in morning, I will make my way to Aso - want to explore the part of Aso (Kusasenri-ga-hama) and also any peak to get nice panaromic view. Before making my way to Kumamoto in later afternoon probably around 4pm.

How do I make my way to Kusasenri after alighting from Aso station? Or if I were to take a bus from Beppu, where should I stop and how should I make my way to Kusasenri. There are loads of info in the internet but I am somewhat confused.

Latest by what time should I be at Aso station to take the train to make my way to Kumamoto Stn?