Parents often ask us what their child should do with their non-writing hand while practicing handwriting. It’s a great question, because it really takes two hands to write properly.
The non-dominant hand should be used to stabilize the paper on which the child is writing. Doing so also helps maintain proper body position for writing: upright with both elbows and wrists on the table and feet positioned directly in front of the writer’s chair.
Making the hands work together during handwriting is a basic component of coordination. So, make sure that the non-writing hand is part of the activity too.