Introduction: hook your readers and tell them why they should read your post (paragraph)
Topic 1 (H2)
Content that belongs under your first topic (could be short paragraphs, bulleted list, images, etc.)
Topic 1A (H3)
Content that falls still falls under topic 1, but is also further sub-divided and distinguished from the content under the other sub-heading (could be short paragraphs, bulleted list, images, etc.)
Topic 1B (H3)
Content that falls still falls under topic 1, but is also further sub-divided and distinguished from the content under the other sub-heading (could be short paragraphs, bulleted list, images, etc.)
Topic 2 (H2)
Content that belongs under your second topic (could be short paragraphs, bulleted list, images, etc.)
Conclusion (H2)
Content that concludes the page (could be short paragraphs, bulleted list, images, etc.). As this is a conclusion, you might also want to consider adding a call to action (link, button, etc.) to direct users to the next step.
Explanation
Notice how every heading is nested under one that is a level above it (ex. every H3 is under an H2). This is necessary to maintain proper page hierarchy and organization.