Is this a standard envelope or one you designed yourself that needs glue? I don't
know how you will print on the inside of an envelope unless you take it apart. As for
a homemade envelope, insert a page, copy from the first page the outline of your
envelope and type away.

Templates
http://www.ruthannzaroff.com/mirkwooddesigns/interenvelope.htm
http://desktoppub.about.com/library/weekly/xenvtemp.htm
http://www.ghh.com/html/ghhcom_frame.html

There are many others, do a search.