Set a ratio of graph boxes on your paper to the actual height and width of the glass. Draw the cartoon characters out and determine their defining features. Find some aspect of the characters that will be exaggerated and made a recognizable part of their identity.
Wash your glass with warm water and dry it off with paper towels. Tape off any ledges or edges you don't want sullied by paint. Square off the part of the glass you will paint with tape and mark measurement lines to help transfer the paper drawing.
Dip your brush into the paint and lightly wipe it onto a paper towel to remove drippy excess. Paint on the glass in smooth, single strokes, avoiding adding unnecessary layers to the picture. Spray drying acrylic with water if you need to make color alterations. Chip away at dried excess paint with a razor blade.