Look for materials that relate to the reason for which you are writing the essay. Evaluation of sources is the final step. You want your argument and your synthesis to be as strong as possible.
Before settling on the thesis of your synthesis analysis essay, conduct a thorough analysis of the relevant materials. Develop each main idea thoroughly. The purpose in writing an explanatory essay is not to argue a particular point, but rather to present the facts in a reasonably objective manner.
See Forming the Thesis Statement Section. Double-check to see that sentences are adequately varied in length and style, and that there are no fragments or run-ons.