What is the easiest way to go from Hong Kong to Guangzhou Airport?

What's the easiest way to go from the city of Hong Kong to Guangzhou airport? Is there any shuttle bus to there?
Should i take the train to Guangzhou and then the bus or should i take the bus from shenzhen?
Have in mind that my flight is at night and i think that buses stop at 21 i think from Shenzhen and downtown Guangzhou...