There is no direct mean of transportation from HK to Yangshou, as far as i know. The fastest way is to fly from HK to Guilin .from Guilin take a bus to Yangshou (about one hour drive).
The flights from HK to Guilin are very expensive (about 200 USD). if you still want to take a flight you can cross the border to china and then take a flight from the border town of Shenzhen, it's much cheaper (70-80 USD).
another option is taking a luxury bus from Shenzhen straight to Yangshou, IMO it's the best way. the bus departs at 6 pm and gets to yangshou by early morning. The luxury buses are equipped with sleepers so you hardly feel the 10-12 houres drive, costs around 250 HK$ (30 USD).
I know that there is an option to go to guilin by train but havn't got details about it.