'Having dwelt there, till their (good) works are consumed, they return again that way as they came, to the ether, from the ether to the air. Then the sacrificer, having become air, becomes smoke, having become smoke, he becomes mist,

In plain words: They stay on the moon until the reward of their good deeds runs out. Then they come back the same way, through sky and air, becoming smoke and then mist.