Although you can make miniature and model windows from glass, if there is a possiblity that a child will be playing with the dollhouse or model, it is better to use sheet plastic for your glazing material. If possible, choose a plastic which will not droop when placed across the widest section of the window, and use a UV resistant plastic to ensure it will not frost over or yellow if exposed to sunlight. For 1:12 scale windows I use 1/16 inch plastic. There are a wide range of available types which are suitable. Most 1/16 inch plastics can be scored with a sharp knife, then snapped along the score line. Edges can be sanded (wear a dust mask).
Measure and Cut Your Plastic - before measuring and cutting your plastic, check that your window is square and true. If your window is slightly out of square, you may need to make paper pattern of the opening so that you can get an accurately cut piece of plastic to fit in the window. Cut your plastic so that it is 1/16 inch smaller than the opening in both directions (width and height). For smaller scale buildings, you may need a smaller gap. I use 1/2 the width of my trim (which is 1/8 inch square) as the amount of gap I can leave to allow the window to flex slightly. Leave any protective coating on the plastic as long as you can through the building process. When you have cut your plastic, test fit it into your finished window frame.