There are two buses from Guangdong Provincial Bus Station (near Guangzhou Railway Station) to Changsha, you can take the bus to Changsha and transfer bus to Liuyang.The bus leave at 20:30 and 21:30 and the latter one passes by Changsha, you need to get off the bus on the way. They are all overnight buses.

But how far is it? how long does it take to reach Liyang from Guangzhou? How do i make an advanced booking??

Hi Mr. Murali, in fact, I don't think it is a good idea to take a bus from Guangzhou to Changsha and then Liuyang. The bus will take you about 15 hours on the way, suffering.

At present, there are over 50 high-speed trains departing from Guangzhou to Changsha, taking only 2.5 hours and CNY333. Then, you could go to Changsha East Bus Station to take a bus to Liuyang in one hour.
You could use the searcl tool of China Trains(https://www.travelchinaguide.com/china-trains/) to get the details of trains from Guangzhou to Changsha.