EXPLAIN "Cassio's a proper man. Let me see now:/ To get his place and to plume up my will/ In double knavery."
I am explaining how Iago is killing 2 birds in 1 stone by destroying othello and desdemona's relationship and cassio's position with this quote
This quote, which occurs in the final scene of Act 1, shows the beginnings of Iago's plan to get back at Othello. In reading this soliloquy, we learn that Iago hates Othello not just because Othello promoted Cassio to the position of lieutenant, but also because Iago suspects that Othello has slept with Emilia. He admits that he knows "not if 't be true," but will assume it is to justify his own actions.
The first part of the quote you're referring to ("Cassio's a proper man") is actually an acknowledgement of Cassio's good looks, as in this case, "proper" means "handsome." This is significant because, as we learn just a few lines later in the soliloquy, Iago will "abuse Othello's ear/that he (Cassio) is too familiar with his (Othello's) wife"--meaning that Iago tell Othello that Desdemona and Cassio are having an affair. Iago determines that because Cassio is so handsome, Othello will be more likely to believe that Desdemona is in love with Cassio.
By telling Othello this story, Iago will presumably win Othello's trust, and will cause a divide between Othello and Cassio at the same time. Obviously, then, "to get his place" refers to Iago's hope that once he tells Othello of these fabricated infidelities, Othello will dismiss Cassio as lieutenant and promote Iago in his place. (Further, "plume up my will" is typically footnoted to translate to "put a feather in one's cap" or "glorify.") Thus, Iago, a master manipulator, will be able to "kill two birds with one stone."
This is a fairly devious line. The part upon which I would place focus would be the second half. The idea of "get his place" is a direct reference to being able to assume the position that Cassio received which he perceived as being rightly his. Certainly, this aspect of coveting something that is seen every day is not something new and is not the basis for his deception. This comes in the following idea of "plume up my will." Iago understands that it is not merely enough for him to bring shame to Cassio, rather he must do so while making himself look great. The combination of seeking to bring others down while simultaneously raising his own stock is where I see Iago as downright brutal in terms of his machinations. The premise is reflective of the complex and duplicitous nature of human beings, and of consciousness, in general. Iago understands that one does not win solely with another person's loss. Rather, they must appear to be better than that which lost so that individuals can gravitate towards them. The idea of bringing someone down is nothing new, but where Iago excels is in the idea of being able to bring himself up in the process. I see the "double knavery" as going after Cassio and also being able to bring himself up in the eyes of others.