Mine's pretty manual but it works:
1. Type the seed into Google and note the autocomplete suggestions, then add a, b, c... after it to get more.
2. Open People also ask and keep clicking, it keeps expanding.
3. Read the related searches at the bottom.
4. Search the topic on reddit and forums and copy the exact wording people use in their questions.
5. If you already have a site, Search Console queries with impressions but bad positions are free keyword ideas you're half-ranking for already.
Google Trends to check it isn't seasonal or dying, and Keyword Planner for rough volumes if you have an ads account (it gives ranges unless you're spending). Don't throw out stuff showing tiny volume. Free tools under-report long tail terms badly.
