What is the best way to arrive to the airport to be on time for the flight? Flight at 8:20AM from Beijing capital

You need to be at the aiport at least about an hour before the departure.
Generally speaking, the first shuttle bus from the city proper area is around 05:10, so I think you can take the shuttle bus to reach the airport on time.
Would you mind tell me your exact place, so that I can help you to check whether the bus is suitable for you or not.