There is no bus service from Guangzhou to SHenzhen Airport. That is one way to get to Shenzhen Railway Station first and then take airport shuttle bus to Shenzhen Airpor then. However, there is a detour for Shenzhen Airport is closer to Guangzhou. Another option for you is to take bus from Guangzhou to Dongguan South Bus Station. Shenzhen airport has its city terminal in Dongguan, just in this bus station. Then you can take airport shuttle from Dongguan to Shenzhen Airport then. Hope this helps.