Keep working with the tutorial. Simply copy the date field for a future date and change the delay. That field, as written, does not have too many switches. These things are complex and it is easy to have a typo cause problems.

On a side note, you should be sending your letter as pdf rather than as a Word document if sending electronically.

Your letters should be templates, not documents. When being prepared, you create a new document based on the template and use the CreateDate field as the basis for computations. Templates in Microsoft Word