In this video, the teacher guides students through the process of writing an introduction that previews the main ideas of their text. They provide practice exercises and examples to help students understand how to identify the main idea of a paragraph and use it to create a strong introduction. By incorporating the main ideas from the body paragraphs, students learn how to give their readers a clear preview of what to expect in their writing. The teacher emphasizes the importance of organizing information and providing a clear structure for the reader.