Clean the glass piece thoroughly using rubbing alcohol dampened cotton balls. This will strip the glass of body oils and other buildup that could affect the application of the paint.
Buff the glass dry with a soft cloth.
Apply a thin, even layer of surface conditioner on the entire glass piece that you want to paint. Allow the conditioner to dry completely. Do not worry if the condtioner causes the glass to look a little cloudier and filmy -- this is supposed to happen and the film will fade.
Dip a paintbrush into PermEnamel glass paint. The paint comes in a wide variety of colors and is available for purchase in craft stores and online.
Glide the paintbrush across the conditioned glass to create the design of your choice. You can choose to make a simple design using a minimal amount of paint, or you can create a larger, more detailed design. Use the paint as you would regular paint, cleaning the brush thoroughly with water in between colors.
Allow the glass piece to sit for 10 full days before using it or washing it in a dishwasher.