Mirrors are made out of glass. It's the backing that's applied to the clear glass that creates a reflective surface. Since the material is identical, paints that you use on mirrors can also be used on glass surfaces. The best paints to use when painting on a mirror or glass are oil-based or enamel paints. These paints are sturdy enough to stand up to the silver backing processes used to turn glass into a mirror surface (when properly prepared) and transparent enough to be used to create a stained glass effect when held up to light.
Clean the glass surface you intend to paint on very thoroughly. You'll create irregularities in the painted surface if there are any particles or stains on the glass. Dry it using sweeping strokes with a lint-free cloth. This will help eliminate any smears or streaks when drying.
Plan out your design. Create a template with the same dimensions as the glass sheet if you intend to make a complicated pattern. Attach it to the back of the glass with a low-tack masking tape. For a simple block pattern, mark off with the masking tape where you want the different lines to be placed. This will also help you to create straight lines.
Paint one colour at a time. Use long strokes to minimise their appearance as the paint dries. Allow the paint to become dry to the touch before adding an additional colour. If the design that you're painting has several layers of colour, start with the foreground colours and move to the background ones.
Remove any masking tape you've used to mark lines or hold templates in place.
Allow the paint to set. Even though it may be dry to the touch in a few minutes, the painted surface is fragile until it has completely dried. Leave it for a few hours, especially if you've used several layers.
Clean the back of the glass sheet a second time to be sure that no residue has been left behind and bring out the colours of the paint when viewed through the glass. To better protect the painted surface, it should be placed toward the wall when displaying the glass.
Cover the entire back surface of the glass with a metallic silver spray paint once you've finished painting the glass to create a mirrored effect. Thoroughly clean and dry your brushes between colour applications. The paint is more likely to bleed if you use a wet brush.