What is the best way to get from the Capital Airport to Tiananmen Square in the afternoon?

Hi, Kalliopy. You can take airport express to Dongzhimen. The ticket fare is CNY 25 per person. And the duration is about half an hour.
Then change to subway line 2 to Jianguomen Station, take your second transfer to line 1 there and get out from Tiananmen East Station. After arriving, just walk to the destination.