Let's create a Main Document first.  The main document will have the text that remains the same in each letter you will send to the people on your mailing list. You can create a new document or use a previously created document as your main document. 

The text in the main document can be edited or formated like any other text in any word document.  Make sure that the grammer and spelling in your main document are accurate as it will be seen and read by all the people on your lists.