Just a small add to the directions for a newbie (as I was one last summer following these directions). There are escalators up to the bus station opposite the entrance to the tube station when you're in the underground walkway between terminals. When you get to the top, go into the central bus terminal and walk through it (that's where you buy tickets for intercity busses and the like, you're just taking a local bus). When you get out the other side, you'll see the stands 18-20 as mentioned. I found the whole thing to be a pretty easy.

As far as knowing what stop to get off at, I was riding at a not-too-busy time and just asked the driver. He announced the stop for me when we got there. It's almost directly in front of the hotel.