There is some good written info on creating and adding Word templates in the Reference Manual. You can find it by going to Help | Reference Manual(pdf) | Reference | Reports | Letters and Envelopes.
The other way is to click on the Help | Help topics | Index tab and type "letters" [without the quotation marks] into the search field.
Scroll way down to find the paragraph on adding a letter into the templates in item #8.
If you would like more detail than is given in the manual, give us a call and we can walk you through it.