Ok, Kathy - I have pictures.
Photo 1 is the paper I'm using. I want the stripe to run up the side of the envelope. If I use the paper as is, the stripe is going to go across at a weird angle.
Photo 2 shows the template in place, with the stripe showing thru the window where I want it. I mark my "A" corner, and trace around the template and cut out my square. (What works best is extending the lines with a ruler and cutting with a trimmer or paper cutter; and double check that your final square is true to size. Tracing around the template actually gives you a square a little bigger than you want.)
Photo 3 is the completed envelope. This one is just a tiny bit wonky, because I threw it together in a hurry and didn't double check to make sure my final square was really 8". But see how the stripe is where I wanted it?
Any other questions, just give a yell
I've only done one template for my A2 envies, since that's what I usually make, but this should work for all the sizes.