In a piece of scholarly writing, such as an article in a journal or a transcription of a lecture, the author will present a thesis and attempt to support it with examples from the text and the work of other scholars. You can find this thesis early in the work, usually in an introductory paragraph or an abstract, which is a brief summary of the article. By identifying the thesis, you can ascertain whether the author's argument has anything to do with your topic.
Once you have identified the theses present in your sources, you can begin to examine if there is any common ground between them. For example, a scholarly essay about allusions in "Ulysses" and another about the mythological structure of the novel might have a great deal of overlap, and you can begin to build an argument synthesizing these two sources.
At times, you will come across essays that disagree or appear to be incompatible. You can build a synthesis statement out of conflicting sources as well -- in this case, you will need to decide with which one you agree. A synthesis statement might be constructed by refuting what one source says, and explaining why another source is more valid, such as: While Author A says "this," Author B builds a much more compelling case for "that" in his essay.
Synthesis papers, whether they are explanatory or argumentative are usually structured the same way: the introductory paragraph should introduce the texts being discussed, and what the writer of the paper intends to do with these texts. This statement of intention, which serves as a new thesis, is essential to a successful paper: it outlines why these texts are being discussed and why the author believes this is an important argument. The rest of the paper should be structured to explore the relationship between these two texts, using facts and textual examples to support the claim of the thesis.