I found a "how to" article for aspiring window painters:
To accomplish your mission, you'll need fast-drying, powdered tempera (water-based) paints, which will usually cost less than $2.50 for a one-pound jar or can. (Don't settle for the tiny, expensive containers of premixed liquid tempera ... and, of course, never use any oil-based paint, because?-while it does withstand sleet, snow, and rain?-your customers won't be pleased when they attempt to remove it.)
Purchase a variety of colors ... red, green, black, yellow, white, and blue . . . at least. For painting on glass, you'll have to dilute the powder more than the directions indicate, as too thick a consistency will crack and peel when it dries. Still, if the mix is too thin, the paint will run . . . so you'll want to experiment at home until you get it right. And you can make the mixing job easier if you first put the powder through a flour sifter to get out all the lumps. (Some people even use a blender to stir up large batches of tempera and water.)
Empty plastic margarine or whipped-topping bowls make good paint containers. The snap-on lids prevent colors from drying out overnight, and you won't have to empty and clean them every day. Tempera, however, does have a tendency to thicken as you use it, so carry a jug of water with you for thinning the mixture.
Begin your pane painting by drawing the design's outline with a grease or glass-marking pencil, since?-if you need to make a correction?-it's a lot easier and neater to rub off a thin line than a big streak of unwanted, half-dried paint. (Incidentally, provided the window is well shielded from rain or snow, I find it easier to paint on the outside than on the inside of the glass .. . but, again, practicing at home will enable you to work either way.)
For applying the colors, I recommend sponge brushes (which are available at paint and hardware stores), because they are inexpensive, can be purchased in a variety of sizes, and are easy to clean. To draw very fine lines, though, you might want to try using a small watercolor brush.
Finally, you'll need a stepladder. (A sturdy box might do if you're not reaching too high.)
And that's about all the start-up information that's necessary. By this time next year, you'll probably be able to give me a few pointers on this cash-producing, creative endeavor!
http://www.motherearthnews.com/library/1981_November_December/A_Holiday_Artist
Good luck!