When you arrive at Beijing Airport, please take the shuttle bus Line 2 and get off at Dongzhimen station. The fare is CNY16. Here, you could transfer to the subway Line 2 (CNY2) to reach Jishuitan station. Then, please follow the signs to Deshengmen, where you could take the bus 919 (CNY12) to reach Badaling directly. You could refer to the details at https://www.travelchinaguide.com/cityguides/beijing/greatwall.htm