If my mom is illegal and has a order of supervision, what's the next step?

Without further information, I would suggest that your mother comply with the provisions of her order of supervision.  If she doesn't comply, she could be placed in detention.  As for the next step, that depends on why the order of supervision was issued.  Often such orders are issued when a person has a removal order but cannot be deported at the present time for some reason or when the person has been released from detention and is awaiting a court hearing.