Make sure there’s a pop-up or static box on your website, inviting people to join your mailing list. (But you’ll need to turn the ‘pop-up’ box off when you have Facebook ads running).
Add the link to your Instagram bio.
Set up a welcome email. Or an onboarding series. So an automated email that people receive, welcoming them to your mailing list, when they sign up. Or a series of emails – maybe one a day for five days, giving away tips and advice. And in the last one, mention your course.
If you aren’t already sending out a weekly (or so) email, start. Not about your course, or what you’re selling, just offering a thought/story/idea/recommendation.
Once you’ve done this for a few weeks, try a selling email. But set it in a story.
Make sure your website is GDPR compliant. This is a legal requirement. It’s basically about getting people’s permission before you store their email address (or any other data).