Take the subway to Wudaokou, and then walk 400-500 meters west along Chengfu Road. Wudaokou is the east gate of Tsinghua, and the east gate of Tsinghua faces south. Some to the west of the East Gate are the South Gate. You can walk there or take a bus at Wudaokou. The bus stop at the south gate of Tsinghua is called Tsinghua Campus. Remember, if you see it at the bus stop, go to the south gate of Tsinghua.

I haven't taken the bus, just look up Baidu map by myself, but I still like to take the subway, although I have to transfer twice.