This plugin shows post/page/category/tag/comment/media/user IDs on admin’s edit post/page/category/tag/comment/media/user pages. You can see the ID when you visit an edit post/page/category/tag/comment/media/user page.
Since this plugin hooks on ‘edit’ link, only users with priviledges can see IDs. For example, a user of author role can see post IDs, while a user of subscriber role cannot.
You can find a brief description in Japanese on a book Introduction to PHP for WordPress Users. (written in Japanese.)
Maybe because the installation process is relatively simple, the official website does not write a detailed installation process for the time being. If you encounter problems installing this plugin, you can solve it through search engines or comment below, I will try to help you solve it
Plugin official website address
If you encounter problems in using the ShowID for Post/Page/Category/Tag/Comment plugin, you can comment below, and I will try my best to help you solve the problem