Exactly so. What you're trying to do is flatten the window into a two-dimensional object. The simplest way is to as CGSGuy suggests and make a paper template. Cover the widow with paper and then draw the outline of the window with a Sharpie or whatever. Remove the paper from the window and attach it to a flat surface. Position an object of known dimensions on the template, I prefer a 16x24 framing square but most anything will do, even a 3x3 post it. Try to put the reference object as near to square to the image of the window as possible. Take a picture of the template and reference object and import it into your software, whatever that might be. draw a rectangle around the reference object in the photo. Distort the photo such that the reference object fits into it exactly. Map the photo into the rectangle. Make the resulting object exactly the same size as the reference object. Unmap the photo from the rectangle. You now have a life-size image of the flattened window you can use to lay out the job.
