How do you write a judge and ask for a " sentence imposition"?

My wife was recently incarcerated for forgery .Since it was commited in South Dakota,the judge was pretty hard on her and she recvd 14 months in a womens prison...instead of county jail.I understand after sentencing you can write a letter to the judge asking for a sentence imposition.....or a sentence modification.What would you write to get the best results?