You choose attackers and make blocking assignments regardless of whether it's your turn and regardless of whether the creatures are attacking you. Your choices must be legal within the normal rules for attacking and blocking.
You can decide that a creature won't block.
If the defending player controls a planeswalker, the person who cast Master Warcraft first chooses the complete group of creatures that are going to attack. Then, for each of those creatures, the active player chooses who or what it's going to attack.
If a turn has multiple combat phases, this spell can only be cast before the beginning of the Declare Attackers Step of the first combat phase in that turn.