Cutting in the window was another scary part of the van build, along with the roof fan. You only get once chance to make it right. Luckily, my cutout turned out perfect. If you are planning on installing windows, just be sure and take your time and measure correctly before you ever start cutting.
On the inside of the van, I had to install a frame around where the window would be. I also installed furring strips so that shiplap could be easily attached to the walls.
Before installing shiplap, I added Reflectix to help insulate a little more from the radiant heat. I tried to make sure every gap had insulation in it before sealing it up.