Mirroring is very important. This should be done from the beginning when children are learning "Daily Do". For young children, you can be on your knees facing the student. Your hands and the student's hands are at the same level, with the backs of hands gently touching while you are singing and signing. This helps students correctly line up their hands and makes learning the hand signs a little easier. Students can also mirror each other.
Mirroring can be done at any point during the learning process. Just ask everyone to mirror and repeat what's being learned.