You could take train from Guangzhou to Guilin first. The train takes about 12 hours and costs CNY215 for hard sleeper and CNY383 for soft sleeper. After arriveing at Guilin station, you can find bus to Yangshuo. The bus departs every ten or fifteen minutes between 06:30 and 21:30. The fare is about CNY10 per person, the journey takes about 70 minutes.
Well, there is also direct bus from Guangzhou to Yangshuo. You can board the bus from the Prvincial bus station or Tianhe Bus Station. The bus fare is about CNY150 and it takes about 9-11 hours.