You should really post your question in the techniques forum.
Try slowly applying the Krystal Klear around the frame of the window with a toothpick. Work inward as each pass dries, add another layer until you have the entire opening filled. Wipe away any excess with a wet Qtip. Once completely cured (24 hrs.), you should have a nice window.
I have some myself that I use of making vision blocks and the like, but I have some doubt that it will be able to produce a film large enough for a windshield. Basically, you use a brush, needle or other small, pointed tool and apply a bead of the product completely around the circumference of the opening where you want the window. Once the bead is in place, touch the tool to one edge of the goo and slowly pull it away. The product should "string" with the tool, forming a thin bubble or film. Pull this toward the far side of the opening and touch it to the bead on that side, if you are lucky, it will have pulled the film over the entire opening. It doesn't work every time. You might try to bend a piece of wire into an "L" that is just a bit shorter than the window is wide, then touch the whole length of the L into the goo, so that you pull a large amount away, producing a wide film, which you then anchor to the other side.
Hope this makes sense.
Or try Steve's way instead.....
How large is the opening you're hoping to fill? I would imagine it's about 3mm x 5mm right? If so, I would advise strongly against using Crystal Clear. Why? Because it's going to be fuzzy and blurry -- totally drawing the viewer's attention to "why is the windscreen so funny looking". Not what you're hoping to accomplish, I believe.
Instead, get some acetate (the clear material used in many types of packaging). Cut the appropriate size rectangle. Carefully take a black permanent marker and blacken the edges of the acetate. Then glue it into place using PVA (white or Elmer's) glue.
Well here is a pic of how the windows turned out on this Matchbox Diamond T I cut out acetate for the front windows and used a tiny piece of screen for the rear one which I saw on a pic of a restored one.
Another view of the Diamond T. This was a great kit of an out of the ordinary subject.
Well here are a couple of images of my finished Diamond T and Sherman tank loaded on the trailer
and the other.
Here's a pic of the real one in Europe