Make a list of the main ideas you want to include in your conclusion. Write those ideas on a separate sheet of paper. For example, if you wrote about ancient Mayan culture, you may list gods, art, sacrifice, Quetzalcoatl -- the Mesoamerican "feather-serpent" deity -- temples, corn (maize) and so on.
Highlight or underline a central fact about each main idea listed on your separate sheet of paper. This step will help you make sure you covered all of the facts you wanted. For instance, find one fact about religious sacrifices in ancient Mayan ceremonies and one fact about traditional Mayan jewelry.
Start writing the final paragraph with a thesis statement. The thesis statement points out what you discussed in the paper, and how important it is to the subject at large. For example, you may write that "By studying ancient Mayan artwork, anthropologists began to understand how important gods, time, geometry and death were to the civilization."
Provide a brief summary of what you want readers to "take home" by the end of your essay. For example, if you wrote an expository essay about ancient Mayan artwork, note how religion, a plethora of gold, the use of hieroglyphics and the importance of sacrifice affected the majority of artwork.
Write one or two sentences about what you have achieved in the essay. Do not give a personal stance on the topic, such as "I think that studying ancient Mayan artwork is the best way to learn about the culture." Instead, state that while your research remains incomplete, you will continue to study the fascinating culture.