How to Make Gmail Automatically Send Something to the Gmail Spam Folder

We’ve been saying for ages that the Gmail spam filters are excellent. However there has never been a (easily found) way to have Gmail automatically mark something as spam and send it to the Gmail spam folder, by which we mean something that you have defined as spam, even though the Gmail spam filters may not have. But we’ve figured it out, so here is how to have Gmail automatically tag as spam something you define as spam, and send it to the spam folder.