Reading is not a straightforward transfer of information from the mind of the writer into the reader's mind, but a fundamentally creative activity - this is what makes it possible for literary classics such as Shakespeare to give rise to an endless series of differing interpretations (often referred to as "readings" by literary critics). The creative nature of reading, however, does not mean that readers' interpretations are random or arbitrary, and it is your job as a writer to guide the process of reading by providing cues and by paying attention to the structure of your essay.
It is impossible for a reader to hold in their mind the whole of an essay of several thousand words at once, so as a writer you have to break your argument up into easily digestible chunks. One obvious way to do this is to make sure your paragraphs are not too long, but it also helps your reader to divide up your argument into three or four main sections which are indicated by signposting.
You also have to allow for the fact that a reader will make assumptions, or inferences, about the nature of your argument and the direction it's going to go. You need to foresee these, and either encourage them, if they support your overall argument, or discourage them, if they contradict it, by pointing out why you didn't want to argue along these lines.