Here are two transport methods for your reference:
1)take the airport bus to China Hotel, A Marriott Hotel, Guangzhou and then take the direct bus to HK airport. It costs around CNY220, and takes around 3.5hrs in total;
2)take the airport bus to Guangzhou East Train Station first and then take a train to Hong Kong. After getting to the hung hom station, you can take the airport bus A21 to HK airport.