If you already have a decent amount of followers on Instagram, put up a Story with the Q/A sticker being like "Hey, I'm doing this thing - if you're interested add your email address here." It'll give you a list to start with. I noticed there's quite a bit of networking going on in Substack Notes, so if you find your kind of people on there, commenting on and sharing their/your stuff back and forth seems to go a long way. I also made a QR code for my newsletter (you could put it on a business card or print it on a bunch of other stuff) to direct people right to it.
