Kabuto showed us how edo tensei works. You need a living sacrifice, the dna of the other person and then do the seals. After that is done, the sacrifice becomes the host of the other peson's soul. When that is done the edo tensei body gets put into a coffin. We know the third coffin Orochimaru summoned in part 1 was Minato, but how did he even get as far as getting to the part of the ritual where you put the edo in the coffin, when Minato's soul was trapped in the Death God?