Personally I like to place my business address at the bottom of the page or the footer. To do this go to the floating bar and click on the button, 'Switch Between Header and Footer.' This will open a dotted box at the bottom of the document. You can type your office address here or anything you see fit for your letterhead. Again you can format the text by choosing the font, font size, color, align the text, etc.
But first, let's learn how to create a business letterhead in Word. You only need two things: Microsoft Word and a ready graphic, preferably not wider than 5 inches and higher than 2. In other words, not larger than 5x2 inches. This is, of course, ideal but not necessarily set in stone. Let's begin. Fire up your MS Word. I'm using MS Word 2003 and you might have a different version, but I believe there is not much difference.