1: use luan before laying the tile and
2: I read I should not screw it into the joist since it is going onto the plywood-is this correct
3: How often should I put screw? I was told every six inches and if this is correct, do I do it every six inches on the entire board?
4: Should I leave an 1/8" between sheets for expansion?
5: Is there anything else I need to know