There are several ways to do it.
1. sponsored newsletter content
2. dedicated e-blasts
3. in-newsletter advertising (banners within the newsletter content)
To name a few....
The first two options will likely require you to do some selling to advertisers. You would need to reach out to advertisers to generate interest in your list.
A simple way to start testing monetizing your list would be to sign up with a platform like https://liveintent.com/. They are an ad network for in-email advertising and will pay you to publish their ads in your newsletters. TechCrunch is a great example of a publisher who uses LiveIntent to monetize their daily newsletter.
How much you can earn from your list is a different topic. This will depend on the size of your list and your average open rates.
Hope this helps. I have a ton of experience monetizing email. I'd be happy to chat more.