The simplest way to do this is to create a template for your envelope. Set the paper size under Page Layout. Note that most envelopes are printed in Landscape orientation.
Generally the information typed on an envelope is positioned using a text box or a frame.
Change Envelope Layout
Templates in Microsoft Word
The button to add to a document is on the Envelopes dialog. See screenshot. This is perhaps the simplest way to do it, but you will have a blank page following if you do this.