If you would like to use your own letterhead for documents generated in PlainStaff, you can upload it in the Admin area under "Document Layout".
To do this, you need an image of your letterhead in JPG format. Simply save a blank Word document containing your business letter template as a PDF file (File -> Export -> PDF). You can then convert this PDF document into a JPG image using the free web service pdf2jpg (https://pdf2jpg.net).