Iago is a masterful manipulator, and Shakespeare includes more than five examples of Iago planting seeds of doubts in Othello's mind, but below are five key suspicions.
1. To accomplish his goals, Iago wants to get rid of Cassio; so he first develops a way to disqualify Cassio as Othello's right-hand man. In Act 2, Scene 3, he uses Roderigo to get Cassio drunk and to pick a fight with him, and then when Iago has to relay what "really happened" to Othello, he tells him that Cassio attacked Montano seemingly for no other reason than because he was rash and drunk. Othello fires Cassio on the spot.
2. In Act 3, Scene 2, when Iago and Othello first enter the garden, Iago draws Othello's attention to Desdemona talking to Cassio, and says, "Ha! I like not that." With those simple words he gets Othello to start questioning why it is a problem for his wife to talk to Cassio.
3. In the same scene, Iago pushes the issue farther by reminding Othello that Cassio and Desdemona would have had plenty of time to spend together because back in Venice, Othello used Cassio as his messenger when he was courting Desdemona.
4. Because Iago has already orchestrated a situation where Desdemona will talk to her husband about Cassio, he tells Othello, "Look to your wife; observe her well with Cassio." He knows that Othello will not be swayed with mere words; so he sets up a situation where Othello will "see" Desdemona's unfaithfulness.
5. The most important seed of doubt is, of course, the handkerchief incident at the end of Act 3. By planting the handkerchief with Cassio and causing Othello to notice that Desdemona does not have the handkerchief, Iago not only causes unwarranted suspicion in Othello's mind but also provides the "ocular proof" that Othello needs.