There is no direct bus from Beijing Airport to the South Railway Station. If you prefer to use airport bus, you can get to Xuanwumen first and then change to Bus 102 to Beijing South Railway Station. Or get to Fangzhuang and then shift to another city bus or taxi to the railway Station.
Subway in Beijing is a good choice. You can use Beijing Airport Express Train(https://www.travelchinaguide.com/cityguides/beijing/transportation/airport-express-train.htm) to Dongzhimen, change to Subway, Beijing(https://www.travelchinaguide.com/cityguides/beijing/transportation/subway.htm) Line 2 and get off at Xuanwumen, then change to Line 4 to Beijing South Railway Station. Beijing Subway is cheap, only 2yuan without limitation on transfer times.