The question will always say: "Which choice best states the main idea of the text?"
Main Idea questions are not quite the same as the Main Purpose questions we covered under Craft and Structure. While Main Purpose questions are more about what the author is trying to accomplish with their passage, Main Idea questions are more about what the central focus of the passage is. The overall approach will still be fairly similar, however. Carefully read the passage, one sentence at a time, and try to summarize the gist of what each sentence is saying in your head. If it helps you, use the highlight or annotation tools available in the app. After summarizing each sentence, try to summarize in your own words what the passage as a whole is about. Once you have a good understanding of the passage, go through the answer choices using process of elimination, looking out for anything that might make and answer incorrect.
Accurate, but not the main idea
Some wrong answers may accurately reflect what is in the text, but only cover a relatively minor point or detail. The correct answer should not just be accurate, but should be the main idea. If two answers both seem accurate based on the text, choose the one that seems to cover the passage more broadly or seem like a more important point.
Steals language from the text
Watch out for answer choices that use exact words from the text, when they could have been paraphrased. These reused words are often there to lure you toward thinking it's reflecting the text. Don't rule it out just because you notice it taking words from the text, but be wary of this trap. Generally, the correct answer is going to paraphrase the text, trying not to pull too much language directly from it.
Seems plausible, but not directly supported
Watch out for answer choices that sounds like they very well could be true, but there isn't actually evidence in the text to support it. You shouldn't have to apply outside knowledge of the world or subject matter - all the evidence you need should be in the text itself.