Then I'd recommend a couple things. They're numbered, but that's just for organization-these aren't in any particular order of importance, or what to do first-last.
1. Create a website. Have people review that website, both martial artists and non. Hire an SEO analyst/specialist to help you out there as well.
2. Research prices of other online schools, and figure out how you want to do it.
3. Reach out to people you've trained with in the past, see if they'd be interested in learning from you. Possibly offer them discounts; class culture is important even online, so they'd be helping set that. It'll also help you retain new students (someone coming in as the only student vs. even 5/6 students can have a very different impression).
4. Join fb groups focused on the styles that you practice. With those groups, don't just start spamming your group-get involved with them, get people to accept you know what you're talking about, otherwise they won't want to check it out. Once they have, then mention your lessons, but even then, don't go overboard.
5. Create a blog with a newsletter, sending people info about your art, advice on certain things, etc. Have a (e)mailing list people can sign up for. I'm not sure if that actually works, but enough people with successful online "schools" have done it I assume it must.
6. Create a youtube channel. This goes along with the online presence thing from the other ones. People will want to get their money's worth.
7. (This one I'd do if you find yourself struggling) Put a beginner's course on udemy for cheap. People might sign up for that, then be interested
8. Create a discord and/or slack for your school. It'll be a way to give students a sense of community they otherwise might not have online.
9. Network. Take advantage of your friends in different areas, reach out to them. See if they'll advertise to their friends, and see if any are willing to join. But don't do it in a way that's spamming them-that might work if you're trying to sell essential oils or tupperware or something, but it won't get you a consistent student-base.
The below actually are more important than the above (to me), but more conceptual so I'm separating them to a different group. Again, numbering is just for organizational purposes.
1. Be patient. With no students, and no "base" to call your own, this is going to take you a while. Good news is you won't have too much overhead (pretty much just zoom membership, your camera/microphone equipment, the cost of a website, and an SEO analyst if you choose to go that root), bad news is, it's a lot tougher to advertise and get students when it's not something they can just wander into.
2. Know why you're doing it. Is this something that's money for you, or passion? Why do you want to open a school instead of just learn, or teach at someone elses? And why are you doing it online rather than in-person?
3. Know what you have to offer. There are plenty of online schools out there, for different arts, if you were talking to me in person, what would you say to convince me to choose yours? What's your elevator pitch, essentially?
4. Know your format. Are you doing nightly zoom calls? Are you sending out dvd's and having students send back dvd's to evaluate? Are you doing belt testing, and if so, how are you evaluating ranks? Will you be offering any sort of in-person seminars to go with it?
5. I included this in 2, but feel it should be it's own point. Know why you're doing it online, specifically. Know the drawbacks of it, and the arguments you will hear against it. If you search through this site something like "online learning" you'll find plenty of arguments about its merits.
I'm avoiding stating anything myself of online vs. in-person, and those questions in the second group are primarily rhetorical: You can answer them here if you want, I'm sure you'll get feedback on those answers which might help. But they're for you to think about as you create your school. And your answers can change as you progress.
Let me know if any of that needs clarification.
