***** NOT A TEACHER *****
(1) I don't think you have to write a letter.
(2) I think the Department of Motor Vehicles has forms for you to fill out.
(a) Probably (I think) the Department prefers that you fill out a form. It is more official that way.
(3) I don't know where you live. In some states, you must report the sale in 5 days.
(4) You can get the form:
a) on line.
b) call the DMV and they will mail one to you.
c) go to a local DMV office and get a form???
(5) It is REALLY important to make sure the DMV knows that you sold your car so that you are no longer legally responsible.