Which bus should I take from Wudaokou to Beijing airport?

Hi,

I am in Beijing and would like to take the shuttle bus to the airport from Wudaokou. Which bus should I take? Where do I actually buy the tickets for these busses and where are the stops in those areas? I'm worried that I won't be able to actually find the bus or the place to buy the tickets unless I have very detailed instructions.

Thanks!
Rachael