When you arrive at Guangzhou Airport, you could take a shuttle bus to Guangzhou Railway Station or Guangzhou East Railway Station in one hour. Then, you could purchase a train ticket on the spot. It will take you about 70 minutes to get to Shenzhen by a bullet train.

Here is the Guangzhou airport shuttle bus:https://www.travelchinaguide.com/cityguides/guangdong/guangzhou/air/express-bus.htm
Here is the train search tool: https://www.travelchinaguide.com/china-trains/