Click on 'File' at the menu bar and then click on 'Save As.' Name the document. Then, just below the file name is the dialogue, 'Save as type:'. Choose 'Document Template (*.dot).' You'll notice the directory at the top of the dialogue box changes to 'Templates.' Click 'Save' and you're done.
Perhaps a good subtitle would be, "How do I earn a bit of money when I learn how to create a business letterhead in Word." Because you can always find ways to monetize your knowledge (that's what knowledge is for, isn't it?).