How to get to Jiankou Great Wall
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.