Simply save the envelope with the letter/document by using the Add to
Document button. However, the Envelope Dialog in Word is pretty horrible: if
you check the FAQ linked below, it will show you how to make sense of the
Envelope settings. However, as I nearly always use the same standard
envelope (DL) for most correspondence, I simply created a Template and
called it DL.dot. For most other occasions, I use translucent L7160 address
labels and have a template called L7160.dot.