Having made countless window patterns for slot cars and static models of all scales, here's how I do it.
Firstly I apologise for the shite photos, but I just can't be bothered with these bloody digital cameras any more. They're all crap and clarity isn't important on this job. The method is clear and that's all that matters until someone proves to me that anything digital is worth a toss. By crap, I mean it doesn't work straight off, like my trusty old Praktika and Olympus proper cameras always did, without fail.
Anyway, you'll need a pack of Milliput, grey/green. Do NOT bother with any other grade. If you think you need finer than grey/green, you're in the wrong hobby.
Some butter/marge from the fridge.
Make sure the body for which you need the windows is cleaned up in the apertures where the glass is going. Slar some butter round the window aperture and for a little distance around, especially on the inside.
Mix up the Milliput equally and very thoroughly, then squidge it into a rough plate shape and with a wet thumb and fingers push it in the aperture from both sides so that it just sits in there with the outside surface just a little proud of where the glass should end up. Keep the digits wet (spit'll do) and you won't stick to the Milli.
Repeat on tother side, being careful to leave the first undisturbed.
The excess all round the Milliput will be clipped off with end cutters when it's set.
At this point, you can't do any more because they'd all stick to each other inside, if you tried to make all four windows, so now park it under a desk lamp or leave it on the digi box to warm gently overnight, as I will do, because the bloody useless camera has left me in a foul key-hammering mood and I'm not doing anything else tonight. Lloyd, if I hadn't have promised you this blog entry, I'd have taken a sledge hammer to the camera and saved myself any future hassle. It is very rare that I contain my foul temper.