Get a hold of vitreous paint (such as Reusche - many suppliers of ounce quantities). I suggest tracing or shading black.
You need to re-do the part of the cartoon for the face to be as realistic as possible. I.e., remove all the cut lines. You will need just a single piece of glass.
Get a copy (from the library or other place) of Albinus Elskus' book on painting on glass. Peter McGrain has one out too. Most of the others are about cold paints.
You will find out about the techincal stuff from them.
You can also see some things on the blog:
http://glasstips.blogspot.com/Use the subject headings "Glass Painting" and "Paints and Enamels"