Handwriting Problems can be caused by Visual Perception Problems, Fine Motor Skills Problems or Gross Motor Skills Problems -
or of course, not having been taught how to form the letters properly.
Visual Perception Problems
The handwriting course that helps with visual perception problems is
Write from the start
It works best for kids aged 4 - 8.
I highly recommend getting your childs eyesight tested by a behaviour optometrist and doing vision therapy if recommended.
Gross Motor Skills
is the best program to address these problems.
Basically in order to be able to write properly you need enough core strength to sit up straight and enough shoulder strength to control your hand.
Fine Motor Skills
This is problems with being able to use your fingers.
To improve this you need to do exercises for hand strength and pincer grip.
Therapeutic Hand Putty
is what OTs recommend for this.