First, since it's resin, make sure the mating surfaces are flat and clean on the inside prior to painting the body. Many resin bodies are too thick or are messy in the window area so it's good to plan ahead.
Second, you can make windows out of flat plastic sheet. I like to use the white canopy type cements and/or scrap book clear tape to install. Either of these glues works well, the Evergreen one is available at Hobby Lobby.