I know many of you are in wonder as to how itachi know soo much about the uchiha and especially its secret techniques and how to use them. I could comfortably say Itachi was the strongest, if not one the strongest uchiha in the uchiha clan at the time of the massacre. and it's very simple how he knows these things.
Some say he read the uchiha stone with his MS...hmmm, maybe he did but i doubt that would give you an izanami tutorial. but the answer all in one manga page (top left panel):
and being so i can also comfortably say Madara can do everything itachi can plus more.