How does is go convince Roderigo that Cassio is in love with desdemona?

Iago is an expert manipulator, and like Othello, Roderigo is an easy target. He has been in love with Desdemona from the start. In fact, he even gave Iago money to sweeten his support in wooing Desdemona.... hoping Iago or his wife would put in a good word for him. Thus, he is already emotionally distraught over Desdemona's marriage when Iago starts hinting around that she's having an affair with Cassio. This is a case of a broken heart not in any condition to use reason.