How do I get from Hong Kong Airport to Guangzhou Airport. arriving HKG 14:30 and dep. 20:30 at CAN

It is suggested to take the HK bus line A21 to reach Hung Hom Station where you can take the Guangdong-Kowloon Train to Guangzhou East Railway Station.
Get out of the station, you can walk to the CITIC Plaza to take the airport shuttle bus line 6 to the airport directly.

Or if you think it complex, it is suggested to order a pick up service, which is much more convenient, but the fare will be high.