Irrespective of what you write, maintain your focus. Your essay must be to the point and not wander off the topic. For example, if you are writing an essay about the history of Egypt, don't discuss South Africa or other nearby countries; keep the focus of your essay on Egypt.
The preface or the introduction helps the reader understand the theme of the essay. Make sure that your introduction sets the stage for the main body of the essay. Don't make the preface too long; keep it short and snappy.
Thematic essays usually are pretty long and contain many paragraphs. Write a separate paragraph for each topic in the theme. Don't forget to outline the body paragraphs. Make sure that every paragraph you write has at least two evidences. For example, if the two topics in your theme are River Ganges and Nile civilizations, you can use the historical evidence to write about the population growth or the cultivation of the two areas.
A strong conclusion is an important part of a thematic essay. People often restate the introduction in the conclusion, which is a weak approach. Why repeat what's already been said? You can certainly summarize the main points of your essay in the conclusion, but be sure to end on a memorable note; readers tend to remember a conclusion that "points forward" or gives them something to think about.