Cosigned a lease, what can I do when they don't pay the rent?

Garnishment proceedings are only available after the conclusion of a lawsuit, when a judgment is obtained, and the losing party refuses to pay the amount the court ordered him or her to pay to the winning party. In your case, before you could obtain any garnishment, you would have to sue your daughter and son-in-law, win, and obtain a judgment against them. At that point, you would become what is called a "judgment creditor." If your daughter and son-in-law refused to pay you the amount they owe in the judgment, you might be able to institute garnishment proceedings against them pursuant to Virginia Law.  If you wish to avoid having to pay rent for your daughter and son-in-law, there may be other options available to you, depending on the provisions contained in the lease agreement and any co-signer or guaranty documents you may have signed in connection with the lease.