ahead and paint an image on the glass free-hand. you can also use a pattern or a stencil on glass. simply tape the design you want to use on the inside of the glass and then just paint the outside of the glass as the pattern shows.
you usually have to bake the paint on the glass. you put your finished glasses in the oven and heat to the temperature advised by the paint company. once the glasses are done having the image sealed into the glass by the oven heat you take them out of the oven and let them air dry. note that you do not have to do 