how to get to Guangzhou Baiyun airport from Hong Kong by bus

Where are you staying in HK?
Generally speaking, from HK Macau Ferry Terminal, Harbor City, Mong Kok & Prince Edward and Wanchai, you can find cross-border buses to reach Oriental Hotel and Garden Hotel, so you can take those buses to reach the above hotels.
Upon reaching Oriental Hotel, you can take the airport shuttle bus line 5 to reach GZ airport.
Upon reaching Garden Hotel, you can take the airport express line 2A to reach the airport directly.