Take subway line1 at Wangfujing Station, change to line2 at Jianguomen, alight at Dongzhimen.
Then walk to Dongzhimen Outer. There are tour bus936 to Mutianyu there. Because there are many buses936, you may verify it’s the bus936 to Mutianyu before boarding. The bus trip to Mutianyu takes about 2.5 hours while the visiting time is about 2-3 hours. BTW, the tour bus936 will stop operating after November25th. After that, you will need to take other bus936 to Huairou first, then make a change to the Mutianyu Great Wall.