How and when did Uchiha Madara learn Edo Tensei? During the Fourth Ninja World War, Uchiha Madara manage to escape Edo Tensei by releasing himself and put himself in Edo Tensei again. But how or when did Madara knew about the hand seals of Edo Tensei?
It is not really stated how and when Madara learned to use ET.

But it is possible that during the battles between Senju and Uchiha, Madara had seen Tobirama use it. And since ET is not a bloodline jutsu, and Madara as top Uchiha ninja, he copied it with Sharingan's ability.