Book a flight from Haneda Airport to Shonai Airport:
https://www.ana.co.jp/en/jp/domestic

Buy a JR Pass: 
https://www.jreast.co.jp/multi/en/pass

About the Yamagata-Tsuruoka bus:
https://www.shonaikotsu.jp/english/highway/yamagata.html

Buy a Sendai-Tsuruoka bus ticket:
https://willer-travel.com/en/bus_search/miyagi/sendai/yamagata/tsuruoka/?gad_source=1&gclid=Cj0KCQiAwtu9BhC8ARIsAI9JHalgRPmbG9RYYlnkYLwR4C-mLfFdmbQMptNvTZNyI-NhJ6pjxhtvNn4aAiCnEALw_wcB

Bus timetable for the Dewa Sanzan:
https://www.shonaikotsu.jp/english/tourism/haguro.html

Hire plans in the Dewa Sanzan region:
https://en.shoko-travel.jp/