You should take a flight from Indonesia to Xiamen, Guangzhou or Beijing firstly and then transfer to Xian by a domestic flight. From Xian Railway Station, you could take a train to Shenmu directly.
The flight and train schedule could be searched respectively at:
https://www.travelchinaguide.com/china-flights/
https://www.travelchinaguide.com/china-trains/xian-schedule.htm