In Othello, Act 3, Scene 3, Cassio visits Desdemona to beg for her assistance in getting his position back. While he is leaving Desdemona, Othello sees Cassio and questions whether or not that it was Cassio:
Wasn’t that Cassio leaving my wife?
At this point, Iago plants a seed of doubt in Othello's mind. He begins to express that Cassio is sneaking away:
Cassio, my lord! No, sure, I cannot believe
That he would steal away so guiltily,
Seeing you coming.
In this manner, Iago is trying to make Othello jealous. He plants seeds of suspicion in Othello. Immediately, Othello becomes suspicious.
Next, Iago begins to plant seeds of doubt about Desdemona's character. He mentions to Othello that she lied to her father once:
She deceived her father by marrying you;
By this point, Othello is thinking seriously about Iago's accusations. Othello admits that Iago is right:
And so she did.
Now, Othello is becoming jealous. Iago has set the green-eyed monster free. Iago is cunning, crafty. He has planted seeds of doubt and suspicion. Now, Othello is in a position to see what Iago wants him to see. Before long, Othello will be consumed with jealousy until he cannot escape it. Iago has done what he set out to do.