It is recommended to take the subway. Take 652 from South Station to the west of Liujiayao Bridge, take Line 5, Chongwenmen to Line 2, Xizhimen to 13, and get off at Wudaokou. It is conservatively estimated that it will take more than an hour.
If you want to take a bus, take bus No.5 and get off at Wanquanhe Road, then change to 944 at the west gate of Renmin University (note that it is a branch line) and get off at the west of Baofusi Bridge. The bus time will be much slower, not to mention the traffic jam.