Let's keep things really simple to start. Find your list of clients. These records are likely stored in your business software or some home-grown solution. Every business will be different so I can't really give you much instruction here. Just go to wherever you have your client information stored and look for an option to export. Almost always one of the options is "export as CSV file". Select that option and export your list into a CSV.
You will want to upload this list into an easy online system to have an automated email and text message go out which will let them know that you're moving classes from physical to online platform like a Facebook Group so you can build a community for your paying members and allow them to continue to get value from you. We will cover how to do this later.