I have the OEM pano screen, but actually prefer the one I made.
You will be making the marks on the underside of the screen, i.e., the side toward the mirror when installed. All you have to do is measure the screen to determine the horizontal centerline (measure both left and right sides to find the point that is halfway between the top and the bottom). Make a small mark on the screen frame with a fine tip pen or pencil at that point on each side. Once you find that, go up 12 millimeters and make a mark, and down 12 mm and make a mark. Do that to each side. Using a straightedge aligned with the marks (a small ruler works great), mark the screen across between the points. Be careful, so you will be accurate. When you are done, you will have two lines across the screen, which delineate the panoramic format.
I like to use a black permanent fine tip pen, for several reasons: I can remove it with alcohol if I make a mistake; I can remove it if I no longer need the lines; I can see the black lines better than the etched lines on the Bronica pano screen; I can make the lines as thick as I want.
A pencil makes a very nice line, but is impossible to erase completely, and it's not as easy to see.
I often leave the screen in even when I'm not using the panoramic back. The lines don't bother me. One way to lessen the impact of the lines would be to make them dashed, or use a few short lines, instead of one unbroken line.
BTW, be very careful about the top surface of the screen. It looks smooth, but actually is a fresnel lens so it has fine ridges on it. It is plastic, so it's easy to damage those ridges and end up with a scratch, which is impossible to remove, as the ridges preclude polishing. Set the screen down on a soft cloth to cushion it while marking it.
Oh, and welcome to APUG, RedSun!