It will be beneficial to step through How to use Mail Merge before you proceed with this article.
Go to Tools → Mail Merge
Click the "To" button to choose which clients will receive the File Note.
Next, add your File Note content in the main text field:
Click Generate and when asked if you would like to create a File Note, select Yes. Now the File Note will be added to all the clients you have entered into this merge.
Note: This feature is normally used to produce a document to send as a mail merge. Once you have generated the document and saved the file notes, you could then discard the actual word generated document if it is not needed.