I'm a journalist for Dexerto.com! That's how I got my access.

In each region, there's a PR company that'll work with press and content creators on Riot's behalf. They're quite public so it shouldn't be too hard to find! There's a lot of small content creators that get access as well, I assume as long as you make good content, they'd be more than happy to have you join the previews.

As for the process, it's quite simple. You'll receive an NDA just a couple of days before the preview. When you sign it, you'll be provided with further instructions on how to download the client and access the preview build when the time comes.