Start with the search query, not the topic
Before outlining, know the exact question your post will answer. This is the H1 of your post and the foundation of your outline.
Why this comes first: A post titled "How to write a blog post outline" is answering a different question than "what makes a good blog post structure." The first reader wants a process. The second reader wants principles. The content, structure, and depth of those two posts are completely different.
How to define your query: Write out the exact phrase a reader would type into Google before finding your post. Be specific. "How to write a blog post outline" is a clear query. "Blog post tips" is not — it could mean anything, and a post written to it will likely mean nothing.
Do not plan a single heading until you have this nailed down.