I haven't started cutting the windshield yet... but normally I rough in 'carefully' as much as possible, or cut away as much as I can with scrissors, sheet metal shears or exacto knife, depending on what I am cutting and then I use a sanding drum on my Dremel tool to fine finish it. Most of the time I don't dare cut closer than like 1/2" depending on the material and how much curve it has. The final step is sanding by hand to straighten up the Dremel sanding as it is very difficult to cut on a perfect straight line with a drum sander.

What you want to end up with when dealing with any plastic is no corners. All corners should be rounded. You also want no cracks. Either of these is a stress point and will be an easy place for a crack to form at which point it can get longer very quickly.

A hint... If a crack does form, drill a hole at the end of it. This puts a round at the end of the crack and any stress is then radiated out from that circle instead of concentrated at the sharp end point.

This also works with metal. Many body shop fellow would do this on particularly older cars where the metal had cracked. I don't know if it became brittle with age or if it was just less knowledge about making steel back in those years. Maybe a bit of both. I have also seen circular saw blades drilled this way, but I would never recommend doing that as the energy released if it broke would be tremendous. I remember one band saw mill had a top wheel on their band saw break. These are huge wheels but not a lot of RPM. It went through the roof and landed on a car about 1/4 or 1/2 mile away. Now that's on topic if anything ever was!

So, basically first think patients and take plenty of time cutting it using a more difficult method. At the same time think smooth. The end result should be a windshield that holds up for the life of the plane.

OH!!! And I use clear silicon caulk to glue my windshields. It does have to be held in place normally for 24 hours before it is completely cured. Test first, but denatured alcohol so far has not had any adverse effect on the plastics I have worked with. Not rubbing alchohol, but denatured. This is very useful for cleaning. It will remove oils and grease, like from your fingers when handling the part. This help the caulk stick better. It is also really good at cleaning us any extra caulk before it starts to dry. I also use this before painting plastic. I clean the item just before the primer coat. So far my paint work has bonded to the plastic really good.

Meanwhile, I'm taking a short break from hacking up my cowling! Actually, it's coming out pretty nice as cowlings go. I had to enlarge the front of the bottom cutout to clear the spark plug and cap. I also had to cut a bit out right where the front and the bottom come together at that 'step' location as it was barely hitting my head. Not large holes. It looks like it's going to work out really nicely. All I lack now is mounting holes for the screws and the big hole in the side for the carb... and then of course going back over all the cuts to smooth them up.

I made an accidental discovery which might come in handy. If you put Scotch tape along all the seams and try to be neat about it, and then run a fingernail along the seams to sort of mold the tape into the crack... using care in this process... no wrinkles, no overlaps. Then run thin CA on the seams inside, it fills behind the tape and makes it look like one part.

Me, no, I didn't use as much care as I should have, so I have some spots. But it really did a nice job along some areas. This put me ahead on my paint preparation.