Well, I have done all of the following: In all cases I make my windows out of think clear plastic, taken from packaging or made from transparency plastic.
Here, I paint curtains on the "inside" side of the window, and once that is dry, paint over it with black paint, or is some cases paint stuff like a vase and flowers seen through the window between the curtains, etc., and then paint the black.
Here, I simply applied masking tape to the inside side of the windows after they were installed.
This is reinforced tape as used for packaging . . .
Here, I painted some windows and masking-taped others.