Trip from Beijing Airport to the city centre:
The airport is a good place to buy your transportation smart card Yikatong Card, as you can pay with it the Airport Express train, the subway, the city buses and bicycles and even the taxis. You will have to pay a deposit of 20CNY (refundable at the end of the trip) and charge it with about 25CNY for each day you are in the city (don't worry about putting more, because itr is refundable when you return the card).
If your flight arrives at Beijing Capital Airport, it is best to take the Airport Express Train (25CNY, 20-30min) and then connect with the subway or take a taxi to the hotel (if you get off at Sanyuanqiao Station you can take the line 10 of the subway, at Dongzhimen Station lines 2 and 13 and at Beixinqiao Station line 5). Both the Airport Express Train and the subway can be paid directly with Yikatong Card. The Airport Express Train leaves every 10 minutes from 6:35 a.m. to 11:10 p.m. To get there, follow the signs at the airport. If you arrive at terminal 1, you will have to take the free bus to go to terminal 2 or 3. It leaves from gates 3-5 and takes 15min. If you prefer, it is also possible to take a taxi directly from the airport to your hotel (70-90min, 100-150 CNY).
If your flight arrives at Daxing International Airport, you can take the metro directly to Caoqiao (20min on the Daxing Line, which operates from 06:00-23:00) and from here connect with the metro line that best suits you (line 10 or 19). It will take you about 70min in total to get to the center (39CNY pp). A taxi from the airport to the city center takes about 1 hour and costs 150-180CNY.
In any case, if you prefer a private transfer to your hotel, we recommend you book it with Civitatis (about 65€ from Daxing and 32€ from Capital).