Dongzhimen takes the express 9 16 to Huairou bus station. Go to the main station When you get off, cross the road at the traffic lights. There is a special bus to the Great Water Wall, but there is no stop sign. Just pay attention to the sign hanging in front of the vehicle.
The last bus of the 9 16 express is 7 o'clock, so you'd better get back to Huairou bus station before 7 o'clock in the evening.
Ignore those black car drivers. If you have any questions, ask the staff at the bus stop. They will tell you how to get there conveniently.