There are two ways for your reference:

1. Take subway line5 at Dengshikou, change to line2 at Yonghe Palace, alight at Jishuitan. Walk to Deshengmen and take bus919 Kuai or 919Zhida to Badaling. The buses leave every 5-10 minutes. Always remember to take the bus at Deshengmen station, or you may take the wrong bus919 and will never get to Badaling. You can take the bus919 Kuai or 919 Zhida from which you’ll get off back to Beijing.

2. Take subway line5 at Dengshikou, change to line2 at Yonghe Palace, alight at Xizhimen and get out through Exit A. Then you'll see Beijing North Railway Station. You can take train Y567 (09:33) to Badaling. About going back, you can take train Y580 (15:22) or Y588 (19:40) to Beijing and then subway to your hotel. Or you can take the bus mentioned above back to Beijing.