How do I audition for this?

If it's going to keep in the real Harry Potter atmosphere, it will be filmed in England.

And auditions will only be found through good agencies, and available to very well-trained and very experienced actors.

If you're in England and have been attending a good stage school for a few years, and have had loads of stage experience, ask your agent.

If you're not in England (or wherever they're going to film it) and you aren't attending a top stage school, forget it. It's not going to happen unless by any remote chance they hold an open call.

But as there won't be any pre-written characters from the books, they don't need to find anyone who looks exactly like JK Rowling described. So it's extremely unlikely.

And even if they did, remember that for the role of Luna in the films, 15,000 girls turned up for the audition.

Which one? There are a ton of amateur web series and fan-made series shows on the internet. Your best bet to get into one of these is to make your own or know someone who is making it and contact them. Of course you'll have to live where they're filming it, be good enough to get in it, and be willing to work for no money.

IF they end up making a series based on the books in a more professional company - then you'll have to talk to your talent agent. Movies, TV shows and the like are usually not cast through open auditions that anyone can show up at. You have to be invited to audition, usually through a talent agent. And you have to be able to legally work in the country where they are filming. Plus a bunch of other legal and and business issues.

Professional acting is not a lottery. It's show BUSINESS. If you're interested in professional acting you should get acting training and experience and research the industry to understand how things work.

Good luck.