Jeff's suggestion is basically what I used. A couple more hints.
Get on of the window envelopes you intend to use and carefully measure where
the LEFT and BOTTOM edges of the window are. Also measure the height of the
envelope. The USPS is very picky about where address appear on envelopes and
has been known to return mail if even a letter is not visible.
Using these measurements, fold a sheet of paper in a Z-fold pattern. On the
top portion, use the measurements you took from the envelope and transfer
them to the paper. To keep the USPS happy, at least 1/10" to the bottom and
left measurements. Insert the paper in the envelope to see how the marks you
made show up in the envelope. Keep in mind the envelope is slightly wider
than the paper and the paper will move around. Keep making adjustments,
including folds, until everything is in the window. It's time consuming, but
this will help get everything where it belongs when you place the address
field on the master document for a mail merge.