If possible type, double spaced. If you do not type, print legibly, leaving a space between each line. Supply the maiden names of women. Most publications have a set list of abbreviations they use. Let them do it.
Researchers appreciate letters wihtout abbreviations.
Proofread for accuracy, as well as, understandability. You may want to let someone else read it to see if they understand what you are requesting and if you have supplied the information needed to fulfill the request. Include your name and address. If possible, give the name and address of a second person. Addresses change because we move or go south for the winter.