From the customer listing report (Customer Menu option 3) there is a line that says print letter below instead of report. If you check that box and either click on create or select to set the form letter to use. This will print a form letter with customers name and address filled for each customer who would be on the list according to the options you set.
If there is already a file name in the box you can click on create to edit it.
The letters will have the customers name and address printed on them ready to be put in a window envelope for mailing.