On the Underground you have to 'touch in' and 'touch out'. The same applies to 'overground' services where the card is valid. (If you fail to 'touch out', you'll be charged the fare to the end of the line, rather than where you got off).
Buses operate on a 'flat fare' basis (i.e.it's exactly the same fare whether you go one stop or 'all the way'), so where you get off is irrelevant. So, on buses, you simply 'touch in' (and not 'out') on the pad next to the driver. Make sure that you hear a 'beep', otherwise the driver will call you back because your card didn't register. (If you've got good eyesight, you can also see the little display screen that shows how much credit you've got left on the card).
Chris