There is no clear answer to this. Everyone is different, and one author’s content and audience will vary greatly from yours. To be succinct, you will have to figure it out through trial and error.
Adding a link that directs potential subscribers to your sign-up page in social posts, or in your bio is a good start. Moving that sign-up to the top of your web page or using a pop-up will garner attention.
It may be as easy as offering a free chapter or a sneak peek at your newest book upon sign-up. What happens to the subscribers who sign up only to unsubscribe when they receive your reader magnet? Hopefully they will find their way back to you. They were interested enough to sign-up, so there’s a good chance you are on their radar.
Try not to be discouraged by those that unsubscribe. People will unsubscribe if they feel their inbox is too crowded, with nothing personal against you. People will unsubscribe if they feel that the majority of the content is uninteresting and a waste of time.
You have to treat your newsletters like a novel (but not in length). You have to craft them to maximize engagement. Ask questions of your audience, ask for feedback, ask for stories from them. When did they first find your writing? How did it make them feel? What did they think of the cover art? What do they think will happen next?
Think critically about what makes your writing and your content unique and use that in your newsletters. People want to know about you and your work, so tell them.