I measured the width of the window including the few inches of standoff needed for the pocket rod I am intending to use, then measured how long I want the panel to be since it is purely decorative and I don't want it to cover the entire window. (The big window in our bathroom is textured for privacy so we get all the glorious light without the privacy issue). I added a few inches to the width for seam allowance and a few extra at top and bottom for my curtain rod pocket and the hem, then cut my fabric.
Tonight's project is to hang the panel! FINAL master bathroom pictures to come shortly.