How do black holes evaporate? Will black holes evaporate, if they evaporate? When and why do they evaporate? What are the conditions for evaporation? Under what principle do they evaporate?
The evaporation occurs through Hawking radiation. This is a very slow and low energy process. So low that the cosmic microwave background radiation, which is just a few degrees above absolute zero, pours far more energy into the black hole than Hawking radiation takes away. So in principle a black hole cannot evaporate.

With the exception of conjectured atomic sized black holes, that is, as Hawking radiation would be more pronounced then. 

Edit: See this answer on the physics SE: https://physics.stackexchange.com/a/26607/55483
The black hole would need to be less massive than the moon to radiate more energy than it absorbs. Bigger than atomic size, as I suggested.