How does a landlord who has a money judgement garnishtheirformer tenants’ SS income?

With just a few exceptions, Social Security income cannot be garnished.  And back rent is not one of these exceptions.  Social Security payments are considered to be "exempt" income and arenot subject to the claims of creditors.  If there only income is from SS, then I'm afraid that you can't garnish this money.  However, to the extent that your former tenant's have non-exempt assets, you could attach (garnish) those.  That would requite another trip to court.

You will need to obtain a "Writ of Execution" (also known as a "Writ of Attachment" or "Writ of Garnishment") from the court in order to start the collection process. Simply take your judgment to the county clerk's office and fill out the proper form(s) for your MI. Then go to the sheriff's or marshal's office with your writ and ask for a seizure of assets form.  For bank accounts, you need to know which bank the debtor use (but nothing more). To garnish wages, you must know your debtor's employer (if they have one).  If your attempts to enforce the judgment fail, you should consider hiring a professional debt collector or agency.