The text for the Statement Cover letters can be reviewed and edited by selecting Maintenance->More->Letters from the main screen:

You will then see the editor screen (which has spell checking)

Letter Editor Screen

You can also insert 'Data Fields' into the text by placing your cursor at the required insertion point and then pressing the button, this will display a list of all available data fields, simply double click to add it.

Remember, this is just the body text, Onyx will produce the address, title and footers of the letters.  As with mail shots (see Customer Contact ) you are able to add html tags to underline text etc if you wish.