Recommendation for cutting corner of tile?
Score it to half depth with an angle grinder to make the corner.  Note that you do not need to score it so the two lines meet as this might bleed over.   Tap with hammer.   

If no angle grinder you can use a sharp chisel and basically scratch through it with about the same method.   The cut does have to be perfect since you will have a plate over it, although you do not want too much coming out.