During the Assignment Phase, players choose which leaders they wish to assign to which missions.  To assign a leader to a mission, the player takes a mission card from his hand and places it facedown near his faction sheet.  Then he chooses one or two leaders from his leader pool and places them on top of that card.  The Rebel player assigns all his missions first, followed by the Imperial player.  After the imperial player is finished assigning all of his missions, the Assignment phase is over.