Compare and analyze
Find supporting and refuting data
In your literature review, be sure to explain how the evidence in each of your articles supports or refutes your research question/hypothesis.
The main trick is coming up with, or spotting, the sub-themes, and that is simply a matter of careful reading. But if it is done well, and you have taken the trouble to find good sources, then you will find, quite magically, that you have constructed the skeleton (and even some of the flesh and blood) of your essay or research project.
In fact, a good literature review can result in an essay that virtually writes itself.