Where can I find the Page ID?

How do I find an id?

Click on a page (or post) in the admin screens, as if you were going to edit it. Now look at the page url, it should say something like:

https://yourdomain.com/wp-admin/post.php?post=18&action=edit

In this example, my page (or post) id is 18.
You can also see the same url just by hovering over each page or post title on their respective list pages.