Step 1: Anonymously report them to law inforcement.
There is no step 2.
There is no step 2.
Nevertheless, if you want to print something and wish to remain anonymous, it isn't a bad idea to assume that every document that a particular printer ever prints can be linked using the printer's serial number, even if you think that specific printer is safe. Never print anything on it that can be linked to your public identity. Don't connect it to the internet.
You may never know whether there's some sort of steganographic encoding mechanism that targets certain print geometries in ways that you can't detect. There probably isn't. But if you're a dissident or troublemaker, can you take even a tiny risk?