How to tare correctly?

Hello,
I hope someone can help me.

I am trying to tare the BNO080. Unfortunately, I haven't really understood what exactly I have to do. 
I know that the sensor has to be properly calibrated. But after that it stops.
When I execute tare.now(), this usually results in a swap of the axes (e.g. x and y, or y and z). How must the sensor be aligned when I execute tare.now()? 