How to get from Beijing Xinxiang Yayuan Apartment (Wangfujing) to Mutianyu Great Wall?

You may take subway line 6 from the nearby Dongsi Subway Station for one stop to Chaoyangmen, and then transfer to line 2 to Dongzhimen. Use Exit B1 or B2 to reach Dongzhimen Transport Hub. There, you can take bus 916Express and get off at Huairou Beidajie. Then, take a taxi to the Wall at a cost of CNY50~60. The whole trip should take nearly 3h.

Woah...and is this just one way?

Well, the Wall is located in the suburb area. The public transportation is not very convenient. You may choose to charter a car or join a one-day tour.
I personally suggest you contact TravelChinaGuide. They provide comfortable drive and professional guide. I joined a private tour last year and it really left me a unique memory.
Here is their e-mail address: traveltravelchinaguide.com. May help!

Thk u so much Barton fr France and Laurie fr Australia!! I wil charter a car as suggested. Thk u for helping me decide!!