How do you file something on someone's credit report?

The first step if you do not have a reporting account with the credit bureaus is to file a civil suit against them and win. (Be advised that the "and win" part is not guaranteed, going to court is never predictable.)

If you have a judgment the next step is to try and collect, if they will not pay then the answer is to file what is called a "Transcript" against them with your civil court. In NY state at least this involves visiting the court where you won your judgment paying a fee and having them sign a document allowing you to file the transcript. You then go to the clerks office and pay another fee where they will record and report to all three major credit bureaus by name. (You do not need a Social Security Number.)

In the mean time you have to sit back and wait for the person to apply for some type of credit, at which time they will be denied until they have showed a satisfaction of the judgment. You can collect interest from the time of filing to the date of paying, along with the fees you incurred to file the transcript. Once paid you must report that it was paid to the clerk's office, or you will be fined.

(Please note: I am only talking about my specific situation, I'm not sure that it will work in your state or locality. If the above doesn't work you may want to look up and try an information subpoena, where the person who lost is required to send data about where to collect the debt, or they will be held in contempt of court. That is when they will probably know you're serious.)