Write some text on a white board in permanent marker. Then have her trace it accurately with a white board marker. Only the bits that she goes over exactly will rub out afterwards. Self correcting
Do a check first!
Also a basic handwriting book is good, but may be hard to get her interested in completing it. Writing doesn't necessarily improve neatness. It needs to be writing that resets the bad habits.
I have used some free make your own handwriting worksheets ( google!). If the text is interesting, about them or funny and nonsense most kids will happily trace it.
Lastly it's kind of important at that age that they actually want to improve their handwriting. It wouldnt be fun getting a ten year old who didn't care to be neater.