What is the best/easiest way to travel from Beijing to Simatai? Would you book in advance?

According to the official website, you can take an express at Dongzhimenwai Bus Stop. The ticket fare is CNY48/person. This express will depart at 08:00, 09:00, 12:00 and 15:30 in weekdays and it will add one express at 14:00 on weekends. The duration is about 2 hours. The return ticket should be reserved at Tourist Service Center 1 hour in advance. It departs at 11:00, 12:00, 16:00 and 21:00 from no.2 parking lot of Gubei Water Town in weekdays and there will be one more express leaving at 19:00 on weekends.
You need to reserve this Great Wall on their official website. However, they only have the Chinese version, so you may ask a Chinese friend to help you book it.
Well, if you want stay there for a night, you may not go out alone at the deep night. Maybe you may also get a jacket when you go out at night.
Besides, you can also contact the traveltravelchinaguide.com, who has provided me with a nice and comfortable trip to the wall. You can consult them for the detailed information.