Take Bus No.916 at Dongzhimen, get off at Huairou Fangshan (diagonally opposite Huairou First Hospital). There you can find local buses to Jiankou Great Wall. The buses run between 10:00 and 15:00. Sometimes you get confused and puzzled of find the local buses, so you can negotiate with local taxi dirvers with RMB 150-200 for a round trip.
 
Otherwise find a local tour operator in Beijing to organize vehicle.