If you'd like to send a personalized letter to a specific person, then do the following:
Use Personal search, EZ-Query, or Quick Find to locate your desired contact.
Click the Mail Merge icon in the upper right corner of the record.
Click the letter's name or edit icon for your desired letter.
If you click the letter's name, your contact's information will automatically be merged within the letter you chose.
If you click the edit icon, it will take you to Mail Merge tool so you can make your desired changes.