Sadly, there are no direct buses from Guangzhou Baiyun airport to the Hong Kong airport any more. Best low cost solution is to take the Metro from the airport to Guangzhou East Train station, then take the HKG train from there. HKG trains have their own ticket office, look for the signs. Trains are frequently sold out, so consider buying your ticket in advance. Hotel concierges can direct you. If purchasing in person on the day of travel, be prepared for a wait of some hours before your train loads and departs.