Fold a sheet of paper and insert it in a window envelope (remove the
plastic insert if there is one). Trace the outline of the window onto
the paper.
Measure the distances from the edge of the paper to the top and left
sides of the tracing, and write them down. Also measure the width and
height of the box.
In a new Word template, click the Frame button on the Forms toolbar
(this is a little easier to manage than the Insert > Text Box). Drag a
rectangle in approximately the right place, then right-click its edge
and choose Format Frame.
In the dialog, set the height and width to the size you measured. Set
the horizontal and vertical positions to the correct meaurements, both
"relative to Page" in the dropdowns.
If you don't want the edges to print, right-click the edge again,
choose Borders & Shading, and choose None. If you do that, you'll need
to turn on nonprinting characters (click the Show All button in the
toolbar) to see where the framed paragraph is.
More information is at
http://www.gmayor.com/changing_envelope_layout.htm.