First, start big. Get the basic shapes right with large lettering. Ideally you would have something you are comparing to like a stencil or grey printout so you can see the difference between your writing and the target.
After you have the shape fairly good large you can shrink it down. You can take your time getting to that and just make a little progress at a time.
If you find it impossible to shed your current handwriting consider using grid paper to force spacing and maybe try your non-dominant hand.