Explain the main idea as a summary of what a passage relates. Demonstrate this idea with short paragraphs. Read it aloud as students read the same paragraph and state the main idea in fewer words, ideally one sentence. Tell the students the main idea is the topic around which all the sentences revolve. It contains the most important information from the paragraph. The topic sentence of a paragraph is related to the main idea but is not synonymous with it.
Present multiple short paragraphs to students and let them try to state the main idea of each. Remind them that it is not so simple as just finding the topic sentence and repeating it. A statement of the main idea may incorporate important details from the other sentences in the paragraph. But unnecessary details should be eliminated.
Read longer passages, preferably from works that the students are reading as a class. Tell readers to constantly summarize as they read and tell themselves the main idea of a paragraph after reading it. This helps them maintain comprehension of the entire work. Demonstrate this yourself before asking students to try it. Make sure every student verbally answers a question about the main idea at least once before moving to the next step.
Pass out worksheets of numerous short paragraphs for students to work on independently, either in class or as homework.