How can I ship something Media Mail?

First, it <<really>> must be something that qualifies for that handling. You can read about the rules here: https://www.usps.com/ship/media-mail.htm and also calculate prices. The simplest thing is to put the book or whatever in a suitable box or envelope, then take it to the PO and give it to a clerk in order the get the price. Be aware that the PO <<can>> open it to verify that what you are sending is appropriate. = = = = = = The material sent must be educational media. It can’t contain advertising, video games, computer drives, or digital drives of any kind. Media Mail can be examined by postal staff to determine if the right price has been paid. If the package is wrapped in a way that makes it impossible to examine, it will be charged the First-Class rate.

It needs to be done through the post office. They still have to weigh it and mark it media mail. And just like the other person said, they can open it to check its contents. You are not even allowed to put a note in with the book!