What a fascinating assignment! Great idea! Well, to help you I am going to give you some pointers that you need to think about in terms of completing this assignment. First of all, you need to be aware that Cassio is a character whose understanding of what is happening at various points in the play is not complete. He, like so many other characters, is taken in and deceived by Iago, and is manipulated by him for his own ends. Your account would need to pick up on this and express suitable dismay and shock at how Cassio had been used.
Secondly, remember too that if you are going to use quotes, only pick quotes that Cassio himself used or quotes that he was able to hear because he was present. Otherwise, you will need to report what Cassio has heard from other characters.
Lastly, you might want to address some of the questions that have vexed critics throughout the ages. For example, was Cassio romantically interested in Desdemona? Was he just flirting, or was he just being friendly? How would you give Cassio life? Is he a bit of a playboy, or is he a serious man focused on his career and advancement?
Hopefully going through these points and considering the issues that I have raised will help you in your response. Good luck!