Monkey-

Augment is an activated ability that you activate from your hand. To do so, reveal the card, choose a target host creature, and pay the augment cost. As this ability resolves, if the card with augment is still in your hand, put it onto the battlefield combined with the host creature.

You can’t activate augment unless there is a host creature on the battlefield. It doesn’t need to be yours. Note though that if you augment another player’s host creature, they control the combined creature.

You can’t put more than one augment card on a single host creature. Once a host creature is augmented, the host part gets covered up and it’s no longer a host creature.