There is now highspeed way from Guangzhou to Guilin, so you may also take a long-distance bus traveling from Guangzhou to Guilin or Yangshuo.

For traveling with bus, you may take bus at Guangzhou Bus Station nearby Guangzhou Train Station, the bus is available at 09:10 11:30 13:00 20:30 22:30 23:30 in everyday, it goes through Yangshuo, so you may choose either of them as your first station to drop off.  The ticket cost is 170RMB in single way.

There is a website available for Guagnzhou Bus Station, but it is only in Chinese: http://www.sqcz.com.cn

If you insist on taking a bus, you can go directly to the bus stations. All the bus stations can be reached via subways and public buses. In my opinion, you”d better take a bus which departs during daytime. Taking a night bus would be quite annoying, because they stop for many time and they may not go through the highways which are better. They go from other routes to avoid the toll stations.