There are two options:
1. Take subway line1 at the nearest subway station, change to line2 at Fuxingmen Station, alight at Xizhimen and walk to Beijing North Railway station. You can take a train to Badaling.
Train from Beijing North Railway Station to Badaling: Y567 at 09:33 and Y575 at 13:19.
Train from Badaling to Beijing North Railway Station: Y572 at 11:42, Y580 at 15:22 and Y588 at 19:40.
2. Take subway line1 at the nearest subway station, change to line2 at Fuxingman Station, alight at Jishuitan and then walk to Deshengmen. Then take a bus919 to Badaling. Be careful with the privately owned bus919, they will cheat on you. The bus919 to Badaling is green and white. The ticket price is RMB12. If you pay by transportation card, it is rmb4.8.

But I prefer the first one. There are lots of traffic jams on the high speed way recently and it will waste your time if by bus.