I would say that Stephano and Trinculo gain power over Caliban without even trying.  Once they see that they have power over him, they try to keep that power by playing on the fact that Caliban is really pretty ignorant and the fact that he hates Prospero.

When Caliban sees the two men, he thinks that they are gods.  This is especially true after they give him some alcohol.  He does not know about alcohol and gets drunk.

Once he is drunk, Stephano lets him think that Stephano is a god and that he and Trinculo will make it so Caliban will no longer have to be Prospero's slave.

This is all found in Act II, Scene 2.