Start very easy and only gradually make it more and more challenging. Client should act like a picture in a mirror, moving at the same time. Facilitator should do it in a manner so that the client can continuously do it. No sudden movements, your job is to make sure the person can follow along. But make it more and more challenging and faster as you go along.
Facilitator says a sentence.
Client needs to match in turn tonality, pitch, tempo, rhythm etc. First do them one by one until she can do it. That is, keep repeating the same sentence the same way until she gets the tonality right. Then say something else and work on pitch. Finally do all auditory qualities at the same time until she can sound exactly like you.