First, take the subway to Changping Xishankou Station (Changping Line), and exit from Entrance A. Then, transfer to Bus Route Chang 31 (headed for Jiesishi) at the Changping Xishankou Bus Stop. Get off at Jiesishi Station. From there, it's still a bit of a distance to the Dazhuangke Great Wall hiking entrance. You can either walk or take a taxi to the trailhead.