How to order a taxi from Beijing Conferennce Center to Capital International Airport?

Hi, Andi! Lots of taxies are available near the center, you can just take the taxi by the road and tell the driver your destination. It may take CNY65-75.
If you still want to book a car in advance, as I know TravelChinaGuide also provide the transfer service, you can book it through bookingtravelchinaguide.com