i need help on othello

i canèt figure out how does Iago dishonor Cassion in Act 2 Scene 3.
this is my guess: he gets him drunk and then get him into a fight thus disturbing Othello with Desdemona

please help and thank you

Please use these links to help you understand Othello:

http://www.sparknotes.com/shakespeare/
and
http://nfs.sparknotes.com/