Once you have some readers coming to your blog, encourage them to interact with you—ask questions, offer free downloads, share advice and get them to either join your email list or to personally reach out and connect with you.
Learn about their biggest challenges (as related to the broader topics covered on your blog). Pinpoint the ways you can uniquely help them by leaning on your own skills, knowledge, experience. After you've determined the right way to help a handful of your readers solve a real problem they're encountering, pitch them (individually in a very personalized manner either over email or ideally on the phone) on pre-ordering your course and getting direct coaching from you in the meantime while you work on building out the actual course material.
Asking people to actually give you their hard-earned money will validate whether you're really solving a true problem for them. You'll also get live test subjects to help build and curate your course content in real-time with. Follow along with my idea validation framework where your focus remains on pre-selling a course to a small community of early adopters.