How do I paint a window mural?

I’m going to assume that your question is about what materials to use rather than about what technique to use to actually draw it.

If it will be confined to ONLY the window glass and not the wooden part of the frames then acrylic paint would be the least messy alternatine.
Since the glass is a non-porous surface it will peel off easily whenever you wish to remove it.

Be sure to use brushes with artificial fibers (such as Taklon) because it’s water based and natural fibers will absorb too much moisture and swell up.

Just go to the largest craft store in your area and they can help you figure out which size bottles to buy. The smallest ones are usually only two ounces and this sounds like a sizable project.

Have you checked the internet yet? I found a whole list of great how to sites.

I did this in junior high for our local post office. About 6 art students were picked, and we painted seasonal murals throughout the year. We used tempura paints, but I’m sure acrylics would be fine, too. Tempura had to be washed off, while acrylics should peel or scrape off easily. One last thing (and I know this is silly, but just in case) Make sure you paint it on the inside of the windows!