Changing the blank template document

The blank template documents are the options available under "File" > "New".

To add a template document:

Open an existing document ("File" > "Open", or "Ctrl + O") or create a new one with the New button on the Standard control bar or "Ctrl + N".
Make any required changes (these could be changing the page size, adding a logo or adding a message). You can edit the template document just like an ordinary document.

When you are happy with the changes choose "File" > "Save Template" and choose a template name.

To make your new template the default template (i.e. the document you see by default when you first open Designer, and which appears when you press "Ctrl + N") check the Use as Default Template option when saving the template.