In a sustained argument one builds a case, step by step, for a position using reason (arguments) to bring one’s audience to the conclusion one wishes to establish.
One does not simply provide a “running commentary” that amounts to a list of points of agreement and disagreement with source material.
In a sustained argument, then, a progression should occur from a definite starting point (or points) to an end point (your conclusion). You will want to choose your starting points carefully.
Your starting points will provide the basis for your conclusion. You cannot provide support (a basis) for your conclusion by appealing to something even more controversial. Imagine, for example, trying to prove that Picasso was a great artist by showing how similar his work is to that of your dog, also claimed by you to be a great artist. Don’t get me wrong, your dog might be a great artist, but far more people would be likely to accept that Picasso was a great artist than would accept that your dog was a great artist. So, an argument of this sort is not really helping your cause. For this reason you should look for starting points which both you and someone who disagrees with you can accept as true. For example, someone who disagreed about Picasso being a great artist, might nevertheless accept that Rembrandt was a great artist, so this claim could provide a useful starting point.
In sustained argument, between the starting points and the conclusion you must provide step by step reasoning that leads your audience persuasively to the conclusion. Partly, this is about organizing your points toward a common purpose. Partly this is about making connections with logic between what your audience already accepts and what you want them to accept.
Several arguments are presented in the above example, but they are all organized toward one conclusion: Picasso was a great artist. Two main arguments are that innovation is a feature of great artists and that influence on subsequent artists is a feature of great artists. Given that Picasso had these two features these arguments both already support the conclusion that Picasso was a great artist. A third main argument addresses possible objections to these lines of reasoning by pointing out that the combination of these features is an even more certain mark of greatness. Moreover, step by step progression is observable in the above example. The first main argument’s claim that innovation is a feature of great artists is itself supported by the two examples of Rembrandt and Michelangelo (an earlier step). Nor are we expected to just accept that Rembrandt and Michelangelo were innovative, rather we are given examples of how they were innovative (an even earlier step). Giving examples is a way of providing evidence. The same structure can be found when considering the claims about influence. Nor are we just expected to accept that Picasso was influential and innovative. The additional step of providing examples demonstrating his influence and innovation is taken.
Note, however, that the arguments do not have to be organized so that the “earlier” steps are all presented before subsequent steps and the final conclusion. Trying to do that would be very artificial and would not facilitate the audience’s ability to follow the relevance of the various arguments.