what is the best way to get from Yangshuo to Zhenyuan?

Well, there is no direct transportation.
You are suggested to go to Guilin first. Then take a high-speed train from Guilin North Railway Station to Guiyang North Railway Station. The train schedules are from 09:30 to 21:28. The duration is about 3 hours.

After arriving, transfer the train at Guiyang Railway Station to Zhenyuan. It takes around 4 hours on the way. Then take a taxi to get to the ancient town directly.