Using card stock or equivalent, make a template a little larger than the one on the plans. Now using this template trim, it down to fit your plane. The reason for cutting it a little large and trimming is that not all fuselage comes out the same. There are two options in adding a little form to it.
If the plane is finished, make a block that is as wide as the fuselage. for a visual photo thank of the wood stock on each side that the window attaches to and thank of that as a solid sheet across the plane. Attach the window to it and lightly heat it to form the canopy. Too much heat and it will sag and deform. All you are doing is giving it a little form.
If the plane is not finished, you can attach it to the plane and heat it with a heat gun. Too much heat and it will sag and deform. All you are doing is giving it a little form.
Now to make the trim work that is on the top and bottom of the windshield. I used the small black gas tubing and slit one side of it. I applied it to where the windshield comes in contact with the wing. I do not know who makes it, but I bought some rubber trim modeling that I painted and put on the bottom of the windshield.
To attach the windshield, I use small screws from Micro Fasteners. Micro Fasteners have some Very Small Allen head screws that from a few feet will look like pop rivets. One problem I have run in to is, after a little use I have had a few of the screws to vibrate out. Simple fix is to put a little glue in the hole before you put the screw in.
For the side windows you have several options. They can be attached with screws, or small pins. Some Cubs had the windows that fit inside a track all the way around. Some windows were flat against the fus. and other protruded the thickness of the window material.
Upcoming experiment. I have some small yellow tube from Sullivan that came with their Pull Pull system. I will cut small 1/8 long sections and glue them in the screw holes. My side windows are attached the same way.